I tried it out these days. The interesting is not the ROI on mining that is aleays at 17 month at the moment you buy (ghs price is based to btc price), but you can click on different coins, you want to get during mining. So its a kind of exchanging during mining. The other thing as I read here somewhere on bitcointalk, that mining contract, that is at 5 years can be exchanged to 24 hours contract. So you indirectly get back your invest but with loosing 20-30 percent. Seems to make sense when coins are hyping or you don't trust eobot anymore and get a quick pay out. Didn't try that yet to exchange back.