I held off for 3 months just in case, then went ahead and installed v4.1.7 in my nearly 5 year old NV. I have had no problems at all (using Windows 7 and Ubuntu mainly, and also a couple of machines still on Vista as yet).
I've since installed SqueezeCenter v7.5 (I see 7.54 is now available:
http://downloads.slimdevices.com/nightly/?ver=7.5 ; good updating instructions here:
http://logitech-en-amr.custhelp.com/app ... a_id/12125. I find that the system is more responsive, but this may be in part to new firmware for my Logitech players too.
My biggest concern now, having upgraded my drives a few months ago to 1.5Tb Samsungs, is that a USB2.0 port just doesn't cut it for backup any more and rsync performance is no solution. I am unhappy with the lack of USB3.0 in the 4 drive Ultra and I will NOT buy two and sync them nor will I pay for cloud backup I don't want (I do use Amazon S3 directly elsewhere).
When my unit hits 5 years in July I'll see what's on offer. I could see moving my drives to a QNAP box, maybe*. But getting the NV to take a USB3 PCI board would be very nice hack indeed. I really do like making things last a long time, especially things that are supposed to be reliable and long lasting, even if they're no longer frontline tools.
*or maybe a Linux box running ZFS.