I once had a warlock in a campaign that ran from 1st level to 9th level. My observation is a mixed bag. At low levels, the warlock was fairly unimpressive, especially since he was a tiefling (i.e. +1 LA). He hardly ever hit, and when he did, it was nothing compared to the fighter/wizard (to whom I foolishly allowed things like a fullblade and wraithstrike; lesson learned). However, he came into his own around 5th or 6th level and dominated the campaign after that. I realized it had gotten out of hand when he started using baleful utterance on EVERYTHING (much like another poster has done). It got worse when he could summon swarms of wounding bats all the time. I can honestly say that I can't rule out his character being partially responsible at least on a subconscious level for the campaign's death (although my busy schedule probably had a lot more to do with that).
I'm conflicted though. I love the warlock class. It has such an interesting flavor and very interesting abilities. Yes as much as I love it, I have seen it do horrible things to one of my campaigns.
Now I'm not the type of guy who likes saying "no" to his players, and after this experience I imagined it was quite possible another player would want to play a warlock again in the future. I rarely just ban things from my campaign altogether. Even though my campaigns are run almost exclusively in Greyhawk, I still find a way to incorporate almost every player character idea. Though I don't often ban things, I do have a tendency to add a lot of house rules. So I approached this in my usual fashion and thought of a house rule for the warlock.
I'll explain what went through my mind when I came up with this idea. My opinion is that magic users should have some kind of limitation. Many things are not a big deal. It wouldn't ruin game balance to allow a 17th-level wizard to cast magic missile an unlimited number of times per day. The wizard will have much better options 95% of the time and that 5% of the time he doesn't, at least he can do something cool besides firing a crossbow. I applied this same thinking to the warlock and decided his eldritch blast, in its purest form (just the ray damage at range 60') was not a bad thing. So the warlock can use his eldritch blast an unlimited number of times per day. Where I realized the warlock needed some kind of limitation was on his invocations, but I didn't want to limit them TOO MUCH, because otherwise the warlock would simply look like an underpowered sorcerer. I wanted something that encouraged the warlock to conserve his magic for prudent situations only rather than letting him invoke with reckless abandon, but I didn't want to prevent the warlock from using his invocations theoretically as many times as he wants.
Here is the finalized rule. A warlock can use his eldritch blast an unlimited number of times per day with no penalty. This represents the special link a warlock has to eldritch energy and his ability to channel that energy scales with experience. However, a warlock has slightly more limited access to his invocations (including those that modify an eldritch blast), which is based on his Charisma modifier. A warlock can use each invocation he knows a number of times per day equal to his Charisma modifier without penalty. Each additional use of a particular invocation beyond this limit requires a Charisma check (DC 10 + invocation's effective spell level). If the warlock fails this check, the invocation fails, and he suffers an eldritch backlash that gives him a -2 penalty to Charisma and causes fatigue until he rests for 8 hours. This Charisma penalty is cumulative. It cannot result in exhaustion, although other effects can still exhaust the warlock as usual. A warlock can attempt to use his invocations as many times as he wants, however, if he accumulates a Charisma penalty equal to or greater than his Charisma score, he becomes catatonic until he rests for 8 hours. This limitation represents the fact that a warlock's grasp of the unearthly powers he wields is not perfect and that the power of personal presence he uses to channel it wearies with extended use.
I think this gives warlocks a good reason to be concerned about overusing their warlock power. At all levels, the warlock will always have something useful to do in combat by using eldritch blast. On occassions, a warlock will want to use his invocations for additional effects. Note that this limitation has little to no effect on long duration invocations. However, I do not believe these cause as serious of an impact on the game as the instantaneous effects do. I am far more worried about the character who can shatter at will than I am about the character who can walk on walls at will.
Anyway, back to your regularly scheduled thread.