Yes I like Casters myself! I think a weak character that gets to be very powerful as it goes on is a nice idea / mechanic.... IF it lives that long.
In games like this Casters can be OP at the higher levels, and as you suggested the trick is managing the resting system (or time) if it is like D&D with limited spells per day.
Not being able to rest in certain situations, and having significant risks on resting in other areas are limitations to resting that are useful to controlling "OPness"
in games like this. Just playing BGee at the moment, and it can be risky to rest when your characters are out of healing spells, and your guys are hurting! Having quests limited by time would restrict resting however!! (ie. need to be completed by day 3)
There are also certain spells that are possibly OP as well, and balance issues could address this.
My issue is not with the casters, but that melee characters (apart from the micromanaging backstabbing thief), can be a little boring to direct in fights.
It will be interesting to see how it all goes.