Buying a Rex is getting more risky. There are beginning to be some engine failures, not because the engine isn't reliable, but through poor maintenance. As has been discussed the RX8 engine uses more oil than most others and needs regular checking and topping up. However, lots of Rexs got bought by non-enthusiasts, company car drivers etc. who expect to run a car between services without lifting the bonnet. Plenty of these cars have been allowed to run low on oil and engines are now failing further down the line.
The obvious thing is to buy from an enthusiast but unless its a one owner car you'll have no idea what his/her predecessor was like. Its a shame because the car is great and the engine is like nothing else but it sounds a bit like playing Russian roulette with your cash to me.
And before any ex-Rex owners flame me, the info came from a sticky on the Rex forums.