There is, of course, more to a keyboard than simply chucking a load of keys at a lump of plastic. The SideWinder X6 definitely isn't what you'd call ergonomic. If you're a fan of split keyboards (which I can't stand but half the TR staff swear by) the X6 isn't for you. In both regular day-to-day use while working and for gaming I found the X6 no more or less comfortable than my first-gen Logitech G15, indisputably the gaming board of choice of its generation.
In terms of general 'feel' though, the X6 is a much nicer keyboard to use than Logitech's. Each key has a resounding and pleasingly tactile action accompanied by an audible click, which I rather like, that wasn't loud enough to distract while 'testing' the board in both the PC version of Brothers in Arms: Hell's Highway and the Source mod Zombie Panic.
Perhaps more useful in-game is the X6's cruise control function, which allows up to four consecutive key presses to be continued ad-infinitum until the button is pressed again. It's probably worth pointing out that macro functions can't be cruise-controlled. Also of note is the lack of built-in memory, meaning gamers who wish to use macros at, say, LAN parties will need to remember to bring their scripts on a USB drive or re-record them at every new PC.
If I'm honest, I can't say I found the ability to use the numpad as a bank of macro keys particularly useful in any game I played, not being a fan of MMORPGs. If my use of Adobe Photoshop consisted mainly of cropping and resizing images, I could definitely see some appeal, and I'm sure many other applications (and users) could reap the benefits of having dedicated macro keys at their disposal. I should point out, though, that to use these extra features the Intellipoint software has to be installed on your PC. If not you'll be stuck with nothing more exciting than a bog-standard keyboard with a few media keys.
With the SideWinder X6 sitting at the same £50-ish price point as Logitech and Razer's nearest equivalents, the G15 and Lycosa, Microsoft couldn't get away with offering any less functionality - which it doesn't. And as gimmicks go, a removable numpad is definitely of more practical use than an LCD screen or the ability to use any key to activate a macro. Add in the fact that the SideWinder X6 is the nicest one to type on and it's a pretty clear-cut choice.
Microsoft's SideWinder X6 is a fantastic return to form for the company. It feels great to type on, has as much macro-ing functionality as anyone will ever need and the ability to swap the numpad from left to right is a real masterstroke. If you're in the market for a gaming keyboard right now, this is the one to get.