On the other hand, if some people _just_ bought Fusion 4 it indeed is very annoying to pay $50 again for Fusion 5. VMware should have a free upgrade program for recent purchases of 4.
Personally, after using Fusion for 4 years I switched to Parallels. With Fusion, even when Windows is idle Fusion would use up to 20% CPU (on a 4-core), sometimes triggering fans. Parallels seems less CPU hungry and runs a bit faster (though this is subjective, try it first). It also has interesting features, such as, suspending of Bootcamp installations and the ability to use all attached displays when in full screen mode. I don't think Fusion 4 supported any of those; not sure about 5. And Parallels has a $29 license right now for those switching from VMware, so it was a good deal as well.