Magoosh has been around for quite some time now, they have high quality test (I have tried some free sources they provide) and would recommend it. :)
However, you should try free practice tests first before investing. There are a lot of test prep companies who provide free tests, try Preped for instance. They have a LOT of free practice tests coming from different GRE Gurus (Vince Kotchian and Rich Carriero - not sure of the correct spelling though) or test prep company (Nove Press and Greenlight)
Hope my insights help!