I guess it depends on what you want from a practice amp. If you are an actual musician then I suppose all you need is something you can plug in and play. Maybe a clean and a crunchy. I would assume practice for a gigging musician is learning songs and working out parts.
If you are a bedroom musician like me, you may want something more flexible. It adds to the fun to have a variety of tones to choose from and even to try to sound like the original recording. I'm never going to play live so this is my stage.
I bought the Mustang II because it was the most cost effective way to have something as described. That said, I really only use one setting that I customized. It's kind of crunchy. If I want it cleaner I turn the gain down. There are far better options for quality sound for someone like me but the Mustang is probably one of the cheapest and easiest. I used to have a multi-effect pdeal through speakers which probably sounded better but was more complex to mess with and could only really be used in that one spot. With the little amp I can pick it up and move to another room if I want.
I chose the II because I though it sounded better than the I but wasnt overly expensive. When I was looking, it seemed the general consensus was the Mustang was probably the best option in the price range.
No practice amp is going to really be any good compared to quality gear. It comes down to weighing budget vs. expectations.