<h1>Quasits are not going to be as helpful as your warlock thinks they are</h1>

Reading from the SRD, here this is the first sentence about Quasits:

>Although quasits thirst for victory and power as other demons do, **they are cowards at heart.**

When you combine this with the description that you posted above:

>At any time and for any reason, the quasit can end its service as a familiar, ending the telepathic bond.

You get the full picture.

Yes, if your Warlock has somehow managed to get an always chaotic-evil cowardly demon to serve him to the death, then sure, it's pretty overpowered.  However, as a DM you should probably not allow those circumstances to ever occur.  A quasit will work with your warlock as long as it suits them, but the minute the warlock asks them to go into danger the quasit will just leave.  Or say he'll do it and leave.  Or say he'll do it and immediately switch over to the enemy's side and give away the warlock's plans.

In reality, the quasit will probably be more of a hinderance than a benefit to the party.  At a minimum, your warlock certainly shouldn't be able to command the quasit to do anything other than what the quasit wants to do naturally (and being chaotic evil, it's probably not what the warlock wants).