I can't comment on what a move from Windows to OSX will be like, as the last Windows machine I used was about 10 years ago so it would be unfair to compare.
However, as a machine for what you want it's perfect; I probably do a 50/50 split between photo and video these days. The iMac was the only option for me really because I've grown up using macs and feel very comfortable with moving between my works iMac and my MBP at home. It makes sense in that situation.
As a photo-editing machine it's the best I've ever used. Can't remember the spec off the top of my head but it has the 1TB fusion drive, 32GB of RAM, the 1GB NVIDIA graphics option and the 3.2ghz processor.... it's rapid, really rapid. I do a lot of batch exports and can't believe how quickly this thing pumps out hi-res .PSD files. The screen is brilliant and it's just a brilliant machine. It cost an arm and a leg - about £2500 I believe - but IMO it's worth it.
What JonathanRyan says about the ports is kind of correct; I don't mind the USB port layout but the SD card slot is a pain because it's so close to the audio jack for headphones (which are in my machine permanently). I can live with it.
The lack of an optical drive is a pain - I need to buy a Superdrive because drive sharing does my nut in - although I don't think the Apple drives are that expensive, probably £70 or so.
For video it's way ahead of anything I've used before, be it a maxxed out MBP or two-year-old Mac Pro. FCP X isn't the most well-liked piece of software around but it runs very smooth on this machne. Where the other video machines I've used struggled was in loading up projects and events, plus applying complex sets of effects and transitions. The iMac NEVER falters. The screen size also helps when working in Motion and After Effects when I'm doing a lot of keyframe work where you need to have a lot of information readily available and easy-to-see.
Is it user-upgradeable? I don't know TBH, but judging on past experience of servicing and upgrading macs, they may have a few more bit and pieces in a tighter space but in general, macs have always been pretty easy to work on.
I suppose that if you use crossover software like PS, LR, AE then the interface of the software differs very little regardless of operating system. It's when you figure in using Apple's own, OSX-only software that it's either a help or a hindrance.
Are the iMacs worth the money? Many people would say "no" and I can understand why - it's a lot of money to blow on a machine that in terms of on-paper specification, probably matches something in PC terms that is a lot less money. I love Apple kit, I always have and always will, so I don't mind paying the premium myself (or making my work pay in this case) because as a whole, it makes me work well and makes me work happily.
With macs I know that I'm getting a machine that's been designed by people who understand the tasks it'll be used for and therefore they're built to a (high) minimum spec. I've never been totally convinced that anything but the top-spec PCs have been designed along the same lines...
