When mine failed it just weeped. I didn't have any obvious leaking but I had major smell and an extremely hard time keeping the windows defogged. It failed and was weeping towards the top of the core and during a long drive the heated air would actually dry it out somewhat. I picked my stuff up at fcpgroton and also picked up the o-ring kit. I didn't need the o-ring kit and it looks like a major pain in the rump to change the rings out.
You should be able to tell really easy if you need the o-rings, just pull the carpet back and look up at the lines above the gas peddle where the lines come through the fire wall. If they are leaking or weeping then pick up the o-ring kit. If not I wouldn't worry about it.
I agree with BB and doing it yourself. It's really not hard at all and the cost savings is HUGE.
Good Luck
DougA