What it is:
A 5 signet warrior build is one that fills the healing slot, the 3 utility slots, and the elite slot with signet skills.
You combine that with the Major Trait from 10 points in Arms, "Deep Strike" to gain 200 precision.
What it is good for:
Leveling. A 5-signet build gives you some lazy passives so you don't have to worry about utilities. The precision at lower levels will give you absurd amounts of crit % which combined with Warriors' natural ease in leveling and soloing Open World PvE content makes for the easiest leveling experience in the game. You'll want to phase it out around level 60-70 as this is where the build starts to fall flat on its face, as we'll see.
What it is not good for:
Dungeons. And probably pretty bad for sPvP. And WvW. In other words, non-leveling/endgame stuff.
Why it is garbage at higher levels:
Plain and simply, you're sacrificing your healing skill and your 3 utility skills for something that is, ultimately, worthless. (I exclude the elite here since you'll want to have Signet of Rage slotted in the elite slot anyway. You just want to be constantly using it whenever it's off cooldown.)
Assuming a signet build will use Signet of Might and Signet of Precision, a signet build gives, in terms of damage output, 290 precision and 90 power. Since at level 80, 21 precision = 1% of crit chance, this gives us 13.8% crit chance at 90 extra power. That's, really...nothing compared to what other skills could be on your bars.
Why any other build is better:
Quick! Replace that signet of might and precision with For Great Justice!.
Right, so combined with Signet of Rage (which we should be using whenever it's off cooldown), FGJ! gives you what may as well be permanent fury (+20% crit chance)(this IS permafury with 10 points into Tactics) and a hefty sum of might stacks (between 3 and 11 depending on your traits and boon duration).
Edit: I realize now that it is only Permanent Fury if you're using the Signet Cooldown Reduction like I do. Even without it, and without 10 points in tactics for the boon duration, you still have an impressive uptime on fury.
Even with just 10 points into Tactics (10% boon duration), FGJ! has a cooldown of 25 seconds and 3 stacks of Might under a duration of 27.5 seconds.
Okay, so maybe you're not totally sold on that alone. But you also get your utilities to play around with (shouts? banners? swap in stability? or maybe signet of stamina or, if you're best race charr, hidden pistol for those tough fights?), allows you to choose your healing skill should you desire one of the other two, and frees up that major trait for something else. It lets you swap in the elite banner, should the situation demand it (though I suppose you could do that anyway.)
In fact the elite banner is a great thing to have on your bar for certain fights. Examples of this include: most fractal bosses, but especially Jade Maw (eh, why not?) and The Legendary Grawl Shaman (you definitely want it here, dead people here mean bad things), Giganticus Lupicus, and some others where a couple of deaths can snowball into trouble and it is hard to revive people. This is doubly important now that res-rushing has been removed.
Oh and also the fact that signets are an entirely selfish build and provide nothing for your team should you be doing say, a dungeon. "But my DPS is higher, that's providing something!" Yeah, well, banners and shouts can make your whole team's DPS higher across the board. Signet builds lack stun breakers and effective condition removal, as well.
So, if anyone out there happens to be using 5 signets, I encourage you to switch off. It's terribly easy to do. It doesn't require you to largely change the way you play, or even the weapon you use. At the very least you can just throw FGJ!, On My Mark!, and Shake it Off! on your bars. Just do it!
A 5 signet warrior build is one that fills the healing slot, the 3 utility slots, and the elite slot with signet skills.

You combine that with the Major Trait from 10 points in Arms, "Deep Strike" to gain 200 precision.

What it is good for:
Leveling. A 5-signet build gives you some lazy passives so you don't have to worry about utilities. The precision at lower levels will give you absurd amounts of crit % which combined with Warriors' natural ease in leveling and soloing Open World PvE content makes for the easiest leveling experience in the game. You'll want to phase it out around level 60-70 as this is where the build starts to fall flat on its face, as we'll see.
What it is not good for:
Dungeons. And probably pretty bad for sPvP. And WvW. In other words, non-leveling/endgame stuff.
Why it is garbage at higher levels:
Plain and simply, you're sacrificing your healing skill and your 3 utility skills for something that is, ultimately, worthless. (I exclude the elite here since you'll want to have Signet of Rage slotted in the elite slot anyway. You just want to be constantly using it whenever it's off cooldown.)
Assuming a signet build will use Signet of Might and Signet of Precision, a signet build gives, in terms of damage output, 290 precision and 90 power. Since at level 80, 21 precision = 1% of crit chance, this gives us 13.8% crit chance at 90 extra power. That's, really...nothing compared to what other skills could be on your bars.


Why any other build is better:
Quick! Replace that signet of might and precision with For Great Justice!.
Right, so combined with Signet of Rage (which we should be using whenever it's off cooldown), FGJ! gives you what may as well be permanent fury (+20% crit chance)(this IS permafury with 10 points into Tactics) and a hefty sum of might stacks (between 3 and 11 depending on your traits and boon duration).
Edit: I realize now that it is only Permanent Fury if you're using the Signet Cooldown Reduction like I do. Even without it, and without 10 points in tactics for the boon duration, you still have an impressive uptime on fury.

Even with just 10 points into Tactics (10% boon duration), FGJ! has a cooldown of 25 seconds and 3 stacks of Might under a duration of 27.5 seconds.

Okay, so maybe you're not totally sold on that alone. But you also get your utilities to play around with (shouts? banners? swap in stability? or maybe signet of stamina or, if you're best race charr, hidden pistol for those tough fights?), allows you to choose your healing skill should you desire one of the other two, and frees up that major trait for something else. It lets you swap in the elite banner, should the situation demand it (though I suppose you could do that anyway.)
In fact the elite banner is a great thing to have on your bar for certain fights. Examples of this include: most fractal bosses, but especially Jade Maw (eh, why not?) and The Legendary Grawl Shaman (you definitely want it here, dead people here mean bad things), Giganticus Lupicus, and some others where a couple of deaths can snowball into trouble and it is hard to revive people. This is doubly important now that res-rushing has been removed.
Oh and also the fact that signets are an entirely selfish build and provide nothing for your team should you be doing say, a dungeon. "But my DPS is higher, that's providing something!" Yeah, well, banners and shouts can make your whole team's DPS higher across the board. Signet builds lack stun breakers and effective condition removal, as well.
So, if anyone out there happens to be using 5 signets, I encourage you to switch off. It's terribly easy to do. It doesn't require you to largely change the way you play, or even the weapon you use. At the very least you can just throw FGJ!, On My Mark!, and Shake it Off! on your bars. Just do it!