First, I'd wonder why it cracked? If these are numerous small spider cracks on the bottom, then (while not required) it probably didn't have the bottom reinforced or set in mortar (highly recommended for most tubs to make it firm). If it is from dropping the shampoo bottle, that is another thing.
I've never done it. My father had a tub recovered (it was hard to find one thatwould fit where the old one was), so he did that. Basically, that was an entire new (thin) shell inserted and bonded to the existing tub. It looks pretty good. It is hard to get a great long-lasting, durable finish outside of the factory. My unprofessional opinion.