Milltek is ace. Sounds a bit deeper than stock on tick over but give it the beans and it sounds great. It's not even noticable when driving on the motorway for long periods of time.
The car seems to pick up from low revs a little quicker too.
They are about £490 which is quite a lot. I was lucky and got mine this time last year in a Mini2 group buy for £318 so keep your eye out for something like that again. Took me 8 hours to fit it though!