If all you're doing is using daggers and stealthing around for RP purposes, go for it.
If you want to actually do damage, both combat and assassination blow it out of the water.
Edit: As a comparison, Subtlety is a utility tree with some neat tricks, but much lower damage. Look at what each tree brings:
Assassination: Mutilate, higher poison damage, some increased energy regeneration, Hunger for blood (15% damage next patch), 6% increased damage, increased crit.
Combat: 5% hit, 2.5% chance to not be dodged/parried, Killing Spree burst, 4% AP/2% damage, great energy regen.
Subtlety: More combo points*, a bunch of stealth moves which will be used between 1-6 times in a boss battle at most, 15% agility and 10% AP.
*To be fair, Subtlety worked okay because of a bug with Honor Among Thieves where it stacks with other HAT rogues in your party (so you get 3 CP per crit instead of 1, assuming you had three HAT rogues). This lead to insane damage... but I believe was fixed in 3.0.8 (or will be soon). Without this bug, HAT rogues just can't keep up with the other specs.
Edited, Mar 30th 2009 9:49am by LockeColeMA