Well combat spec was extremely nice for leveling up really fast on the rogue, no doubt about that. It is also very nice if you have epic quality swords from MC/BWL and you want to do as much dps in raids.
However when it come to PvP, combat really is only worthwhile if you need to take on only warriors and you have epic swords.
However you will notice this is not the case. You have to set up talents so that you can get combo points as fast as possible or use burst damage via the use of daggers. In large scale pvp you alse need to move in and out of battle all the time, I suggest having 4/5 Master of deception and 4/5 quicker restealth.
cheapshot is a good opener, however mages seemlessy blink out of any stun. In this case the ambush opener is best. having 45% extra crit chance on this is also very nice, same goes for back stab.
Try to use a mod called energy watch. It shows when your energy will regenerate. So what you do is, you start with full energy, just before the tick should go off to regen, you start the ambush, so that you can backstab instantly after that, because your energy regened between the 2, and thus you had enough for the backstab.
Imagine both of those crits for 1900 and 1200, thats 3200 damage in about 2 seconds average. That is what I get with a blue dagger (stupid quest life force dirk from Sunken Temple), some blue gear and still green gear.
Imagine what you could get with epic gear and daggers.
Usually I look for the casters who is out of mana, and sitting down drinking water without a mana shield on. They are my number 1 targets. It usually look like I 1 shot them, but really its about 2 abilities and 3 normal hits that go off.
Only way to beat pallies/warriors is with imp expose armour and using blind + restealth to get 2 time openers on them.
Remember, leather classes are just as soft as cloth classes.
And hunters, well make sure they dont see you first.
The mods I use to help me in combat:
ccwatch - Works for all classes, watch stun durations ect (gouge, kidney, cheapshot) and also on dots ect
energy watch v2 - helps to know when your energy will regenerate, its like a heartbeat monitor
spellalert - really nice to see that heal coming and to kick it, or to see this warlock is busy casting fear, enable Will of the Forsaken so long..