I've had no trouble with iDefrag 2.2.8. I used it recently to defrag and reorder the files on the 1GB Fusion Drive in my 2013 27" iMac. It restored scattered system and application files to the 120GB SSD portion of the drive. As a result, my system runs faster and apps start more quickly - as they should.
I had previously been using TechTool Pro to defrag my drives, until I noticed that it had no file priority options. iDefrag offers a number of these - as the old Norton Utilities did. On a stand-alone SSD or HDD this might not make much of a difference, but the performance difference between the two elements of a Fusion Drive is significant.
As for iDefrag version 5, if you are currently using Yosemite, my sympathies. In case you hadn't noticed, it's still effectively in beta; the OS X 10.10.3 update has just been released to the general public for beta testing, as the first iterations of Yosemite were. Now, six months after it supposedly came out of beta, Yosemite is still not ready for prime time. As a result I won't be upgrading my mains system until I read reports from early adopters that version 10.10.3 is stable and all, or at least most, of the persistent bugs have been fixed.
Because I upgraded to iDefrag 2.2.8 not long ago, and am still running in Mavericks, I will have no need for version 5 any time soon. When the time does come for me to move to Yosemite, I will consider iDefrag 5. In the meantime, iDefrag 5 has arrived late only for those brave enough to have migrated to an unfinished version of OS X. For the rest of us the delay, if you can call it that, has been no inconvenience at all.