How to have a free-standing bathtub resurfaced
Best thing you can do is ask for references.  Call some people who had their tub refinished several years ago and see how it's holding up.

I've had this done a few times and it's a bit of a black art.  They have to be really meticulous with their prep work or the new finish won't last.

Also, you need to be careful what you clean it with after they refinish it!  Abrasive cleaners will destroy the finish.