I think it all depends on what you are expecting and comparing it to.
I haven't had any problems with my V8's transmission at all, in fact I was pleasantly surprised that it wasn't nearly as bad as folks seem to make it out to be. It shifts pretty smoothly, and has a much better feel than the DeLorean that uses the same UN1 box.
For all the doom and gloom that I heard about them failing, it doesn't seem to come up on the forums much at all that someone killed their tranny. I expected it to be a regular post topic, but it never is.
As someone else said, don't expect a dragster, but of course some "spirited" driving is to be expected. The way I see it, if it breaks, it's a good excuse to go with the upgraded "Bell" kit.
However, if you know you will be tracking the car, modifying it with a lot more power, or drive it in muscle car fashion, you might want to go with the upgrade from the start.