I'm loaded with opinions, although I'm more familiar with the RCX version of Mindstorms.
Could you be more specific about where your interest lies? Lego Mindstorms, in general, is a nice entry-level robotics kit. The programming environment is graphical, which is confusing while you're trying to figure out what the little pictures mean. The RCX processor was pretty limited (only three inputs and three outputs) but NXT processor has a lot more capability, including loads of optional sensors, which makes it good for a wide range of learners.