would highly recommend reading the various stickies about locks. To give you a brief answer, locks level as fast as any other character, k maybe a little faster when played well simply because we are basically cloth wearing hunters.
Leveling is easier because when you get your voidwalker and then felguard if you go demo you basically have your own personal tank to take a beating while you simply dot, bolt or drain life.
PVP really isn't my schtick but from what i've seen a good lock is always welcome in arenas, raid, locks are extremely nice for the dots, the extra dps and the ability to summon a player right to where the group is.
In the end yes cloth sucks when you get caught offguard by a stealthed rogue or you pull aggro off your pet, but in the end like any other character the better you are as a player the less problems you'll have just wearing cloth