No experience as an agent, but just from my observations recently buying and selling a house it seems like it would be difficult and probably impractical if you're already employed full time. First off, if you're an agent for say Re/Max you have to pay them an annual fee for office space, the name, etc... and I don't think it's cheap (I think it's upwards of $30K). Second you're at the whim of your clients so if they want to see a house at 9 AM on Tuesday what are you going to say, no?
If they want to call you and talk about x, y, or z or just to bitch during your normal work day what are you going to do, ignore them? I doubt that'll go over very well. Third and probably the most troublesome is the inspections and actual closing of sales, which I'm willing to bet occurs the vast majority of the time between 9-5 on Mon-Fri. So every time you close or there's an inspection you'd have to take off and closing doesn't always go smoothly. Plus you've gotta be pretty flexible around client schedules, which would be difficult while working a normal job.
My $0.02 anyway.