I've done a fusion drive set up on my iMac, 250gb ssd and 1tb hard drive, works really well, boot up and shut down speeds are quick but not quite as quick as my mac book which is ssd and as Craig said is ridiculously quick. There is an argument to just use the ssd for boot up and most used progs and I think this is quicker but I wanted the ease of 1 drive and the benefit of improved speed.
Sent from my Zed using Nangkang tyres front, RE040's rear