Hi I've been using the GMX5 for a week and I must say that for it's price, it's a pretty decent mouse.
Originally I bought it for gaming (CS / SC2 / DOTA) and while it's 'ok', I do feel that it's (light) weight is an issue (at least for me).
The palm grip suits my style a lot, the laser mouse and the 2000 dpi doesn't disappoint either. Ergonomically the GMX5 does feels better than some of the more expensive gaming mouse that I've tried, and it glides nicely onto non-padded table surface as well.
Overall, for it's price, I think that GMX is a very good entry level gaming mouse or for your usual office/work usage -- BUT if you're an avid gamer that's looking for a top-notch gaming mouse, then this Prologic mice isn't for you.
Just for the records, the GMX surely cannot compare with the Lachesis | Naga | Kone, but it does its job nicely. If it has a tad more weight and a fuller palm grip, it'd be almost perfect, for me.