I can at least talk about opening up the Macs, since I do that daily...
iMacs aren't too bad, the only thing that sucks is the glass. All the aluminum models started using a glass panel held on by magnets. The official way to remove it is with two suction cups, then keep it in an anti-static bag. I think with the newest model, since the glass reaches to the edges, you could pry it up. The hardest part is keeping it clean; the LCD creates static, which attracts dust to both the screen and the glass. The inner side of the glass and LCD panel are untreated, so you have to be careful cleaning them. We have a specialized silicon roller, but you could get away with compressed air and a lens cleaning cloth. Try to avoid getting finger prints on anything. Almost all the screws are torx 8 or 10, with some phillips #1, so make sure you have the proper screw drivers.
New MacBooks and Pros are actually really damn simple, especially models without the removable battery. If it has the removable battery, you take that out first, then 8 screws on the bottom panel, otherwise it's 10 screws on the bottom panel. There are 2 internal latches on either side, but they don't hold tight. Most screws are phillips #0, with some torx 6. The new unibody white MacBooks made it harder; they have torx 6 and 8, and phillips #0 and #00. The models without the user replaceable battery have unique screws for removing the battery; small tri-lobe for the MacBook and 13" MBP, 5-prong for the 15" MBP, and a large tri-lobe for the 17" MBP
Since you mentioned it, the Mac Mini is kind of a pain in the ass. The easiest way to open them is to use a grinder to sharpen two putty knives, then you use on on each bottom side to pry it open. The thing is set in two tiers, with the RAM on the bottom tier and the HD on the bottom side of the upper tier. It's just 4 screws to remove the top tier, and a few cables. The HD is a bit annoyingly placed, but not terrible.
I'd say, If you can give up fiddling, get the iMac, but if you can't, you won't be happy with it.