My first 1/8th buggy was a Rex-x. This uses the force .21 engine. I can only say good things about this engine. It starts easy, idles niclely and make good power considering its price. I currently have about 7-8 gallons through mine and it is showing no signs of slowing down. It does like to run on the warm side as someone else mentioned but who cares.
Using this car/engine combo I have beaten some big dollar cars (MBX-5 prospec with nova rossi engine, various Hyper 7's with V-spec's).
For a replacement budget engine I would have no trouble buying another force .21.