I concur with previous poster about cex.io
Started a couple of days ago and it's working and seamless to start.
It's good that you can sell the GH/s so you can recover your investment. Unlike buying hardware asic. I bought from BFL and waited 8 months. The cost of that ended up 40$/GH/s. At cex.io I get about 23$/GH/s (depends on how you calculate different curriencies and which exchange you use) and I get GH/s instantly, not after 8 mos.