Depends on what you consider a pain in the ass...? I usually cross about once a month, and I have been very forturnate not to even get stopped at the border. Others have not been nearly as lucky. Customs acts on instinct, profiles, and sometimes just blind luck. If you do your homework, maybe even email a few of the resident "experts" (Slopain, being argueably the best) on Mex, you will probably have little problem. Note I said probably, there is always a risk!!! Bro's find that out everyday...
If you get caught, it depends on the agents, mood, time, the amount you carry, your attitude. Lots of things seem to come into play here. Customs seems to possess great latitude in what they do!! Usually (note again "usually") if its a personal amount there will be a fine ($500 - $5000) and you will be released. Again you never know they may decided to arrest, and book... Be careful!!!!!!