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I have an Otty and a Simba.
Personally I prefer the Otty which is very firm. On the other hand my girlfriend prefers the Simba which is a little softer (although still firm). They are both decent though.
I'd suggest keeping an eye out on the refurb stores on eBay. You can get cheap ones that have been returned and steam cleaned for about half price (they're often in eBay's 10-20% off deals).
Any issues with them? We got one of the fancy Ikea foam ones when we moved into a new place about a year ago, but I fucking despise it. It's soft and offers no support. Seems to have collapsed. I side sleep and frequently wake up with lower back pain.
GF wants another foam one, but I'm not so sure.