There can be huge differences from product to product.
What is the base made of? It can be anything from really flimsy acrylic to solid cement with an overlay or anything in between.
Does the base go right to the floor? Does it have adjustable legs? Is it just sitting on a piece of foam?
What thickness is the glass (is it even glass)?
How good is the track system? Does everything roll smoothly on a nice ball bearing system, or is it just some crappy plastic wheels that will bind up all the time?
How is the water seal? Are the doors engineered well so that they have little gap, or is the a huge hole that will make your bathroom a swimming pool if you aren't careful?
Generally speaking, you get what you pay for but not always. Look up the brands and models online, get some user feedback (keep in mind that most online feedback is negative no matter how good a product is, use your judgement in this)