Depending on the model*, I definitely think it's worth the cost. I recommend the Space Mouse Classic or; better yet; the SpaceMouse Plus USB. The "Plus USB" is supposed to work with MS Office applications in addition to most (if not all) high-end CAD applications. I must admit that I didn't pay anything for my classic. I won it by simply registering for an AutoCAD online seminar (...I didn't even attend the seminar). That was several years ago when "3DConnexion" was called; "LogiCAD 3D". However since then, I have recommended that my company purchase several. I just submitted a new purchase request of 3 more for new designers I have starting next month. If you're looking for bargains they can be found quite often on EBAY (online auction). In fact there's 3 up for bid right now, but the seller currently states he will ship to US only.
*FYI: We also purchased 3 "SpaceBalls" ...they work OK, but they have all plastic construction; they're not as responsive & they don't have the same solid, heavy feel as the "SpaceMouse".
Regarding the buttons; It's also helpful to have one that resets the default view. You might also consider having three (or more keys) assigned to set the model in the standard orthographic viewing planes (front, top, right, etc). I've found that useful at times.
Since installing the spacemouse I havent noticed any increase in 'Hang ups' but then wildfire always gave a few anyway. For me, these mainly happen when first opening a part but are generally cured by reopening the part (from disk or memory) and will continue fine from that point on.