Yeah the pads have to be broke in, additionally hydros will need bleeding.
However, the brakes shouldn't feel weak (unless they are low end brakes), my BB5s (the only mech worth using (that and other avid mechs obviously) apart from some systems like IRD's Dual Bangers which are less common) felt a little spongy and underpowered but they weren't weak by any means and were still leaps and bounds better than v-brakes and cheap discs (*cough* promax, mech shimano *cough*) when I first got my bike but now that I have them setup properly and have used them a lot they are excellent, what I like about discs is the modulation, I have mechanical discs and even so, I really like how well they modulate over v-brake, I can't wait to try some hydros.
At the moment I have my rear brake set up ultra tight, meaning it will lock the rear wheel with little effort and very little lever pull but will obviously not modulate well, this works like a charm for my trials needs, not a problem though for when I ride other stuff since my front brake is set up to modulate pretty well and since the front brake is all I need then I can truly have my cake and eat it too.