I've heard great things about the Bissell ones....or, you can hire them from DIY shops - £25 for a weekend...worth it if you only want to shampoo once a year or so...
I personally have a Vax one...its okayish providing the carpets are not that grubby to start with and it doesn't seem to make much difference to the high traffic areas. Wish had gone for a Bissell now btw