Nice to see people are still giving unbiased answers to people's questions...
OS 4 is usually hit on by:
a) People who are MorphOS fanboys (if you look really carefully you may just spot on or two in this forum, nay even in this thread!)
b) People who believe anything using AmigaOS has to have a custom chipset underneath it.
c) People who believe the hardware is too expensive and therefore the software must be no good.
Generally, people who have actually tried it have been very impressed by it. I can't talk for MorphOS as I've not tried it, and it wasn't even in the original question anyway! But what I can say is that AmigaOS 4 is definitely worthy of the Amiga name because it isAmigaOS. When AmigaOS 1.3 evolved into 2.04 it broke things, but it was still AmigaOS. When it went to AmigaOS 3.0 it broke other things, but no-one would say that it wasn't AmigaOS. AmigaOS 4.0 had to not only move to a completely new architecture to survive, but it also had to have various new elements added to keep up with peoples demands these days, and these things always come at a cost. Ask Apple, they know all about advancements at the cost of compatibility (remember, Apple's method with OS X was to throw an entire OS 9 installation at any program that didn't play nicely with OS X!).
Don't take my word for it, try it for yourself, and draw your own conclusions, because there's a lot of fanboys out there with no objective opinion. Personally, I think the fact that I've been using (and coding for) Amigas since 1989 and find AmigaOS 4 to be quite incredible speaks for itself - I have 3 OS 4 systems, an A4000, an A1XE (G4) and a Sam 440ep - and I've not looked back.