I started with Ozone 4 about a year and a half ago and upgraded to 5 (while the intro offer was still going). I think 5 sounds and functions better.
That said, I haven't had a ton of plugins aside from the ones that came with my Metric Halo +DSP license and Reaper. Outside of that, I've maybe demoed or bought 10-20 plugs so I don't really feel like I'm qualified to say, "This is killer" or "This sucks".
Personally, I feel like it's an excellent learning tool. It has just about anything you could ask for in a single plug-in but that's a double edged sword. You can go crazy and mess stuff up beyond belief (but IMHO, that's something that everybody has to do for a while). And while it's a little better now, it can be a CPU hog. If you have an old computer, and your host DAW isn't light in it's requirements, you might be better off with individual plugs.
I'm hoping to learn enough to "outgrow" some of the modules in Ozone eventually,.. but I could see using it for quite a while.