Jesus Christ this thread is depressing. There's only a handful of people using decent keyboards.
Currently I'm using the Matias Tactile Pro 3 with my iMac. The Apple wireless keyboard and low profile keyboards in general are **** and horribly unergonomic. Using the Apple wireless keyboard causes my carpal tunnel to flair up. My Filco is on the PC downstairs. Cherry blues. Fantastic to type on.
As for the Matias Tactile Pro 3. It's not built as well as the Filco, despite weighing more. It's got considerably more flex in the chassis when you try and twist it. Also, aesthetically, it's like marmite, you either love it or hate it. Personally I don't hate it, but I'm considering having someone knock me up a custom metal case with much better rubber feet for it to transplant it to. That said the built in USB hubs are great as you don't have to go reaching around the back of your iMac to plug flash sticks in. The best thing about it though is how it feels to type on. I'd never tried Alps switches before (but then I'd never tried Cherry Blues before I jumped in and bought a board with those) and didn't know that I'd like them. But having been using them for a few months, I actually prefer them over the Cherry blues. The downside is that due to the build quality of the MTP3 it's a lot more "clanky" to type on, whereas the Filco is much more "clicky". I do however prefer the action of the Alps. Another weird thing is that all the keys aren't clicky. For some reason they've made the Caps Lock, F14 and clear keys linear. I don't know why, but it's a bit silly. The rubber pads on the bottom of the board are non-existent so it skates all over the place and doesn't provide as much insulation from the desktop surface as the Filco. As a result, if you bottom out, it sounds like it's raining bricks. Thankfully, I've become a light typist since switching to mechanical switches.
There's a distinct lack of mechanical switch keyboards for Apple machines and this is definitely the best of a bad bunch. I paid over £100 for it (including shipping) (and for the Filco as well) and it's definitely not a £100 keyboard. At a push, maybe £60. There are other Apple keyboards, but they're much worse. An example being the DSI SMK-88 which has a ridiculous two usb cable design, one to power the board and the other to power the USB hub. And since there's only a few USB slots on the back of an iMac you really don't need a single device taking up two USB slots.
I contacted Matias to ask about the UK version they were announcing on their twitter a while back. The differences are the @ and " keys will be switched around and it will get the UK style large enter key. If you've been using Apple keyboards for a while, you might as well just get the American layout Matias as I did. The key layout matches the Laptop and Apple Wireless keyboards, the only difference being a single row enter key and the relocation of the pipe and slash key to above said enter key. It means you don't have to get used to switching up typing (again) that much, only training your little finger to locate the enter and pipe slash key.
I also asked Matias about a space saver and they said that they were considering it and if they were to do one it'd be announced possibly at the end of 2011 Q4.