Installation off the DVD takes about 45 minutes, it will ask you if you want a compatibility check and whether you want the 32 (x86) or 64 bit version, this will depend on what CPU you have. Then it takes you to the disk selection stage where you can tell it where to install, after that its half an hour of copying files and a couple of re-starts - all pretty painless.
Also in the installation process is the option to 'upgrade' or 'custom install'. You can only upgrade if you have Vista already, you can't from XP - it needs to be a clean install. Upgrading is not usually recommended anyway as a clean setup will work better in the long run. I'd suggest installing it on a spare hard disk to test it first, also make sure all of your data is backed up.
Although Microsoft tout that it will work on older hardware it still had problems with drivers as expected. Things like sound and network devices that XP finds automatically W7 won't so you may need to manually download the drivers if you have an older machine. It seems to have more drivers for newer hardware.
Startup time is very quick and it seems to assess your hardware and allocate resources accordingly, unlike Vista which just chews a gigabyte of memory before it is ready to do anything. So yes, it is suitable for older machines - it installed and ran ok on one of the orphanage computers which was a 6 year old Pentium 1.8 with 512 megabytes of ram.
It is quite easy to tweak and within a couple of minutes you can turn off all of the annoyances such as UAC, auto updates, and the multitude of Microsoft alerts and messages if you don't want them. The task bar is a lot better and it looks like they've pinched a few ideas from Apple here, rolling over minimised tabs shows a little preview window and general manipulation and switching between them has been vastly improved.
Operation seemed very fast and snappy, unlike XP which has to think about things before engaging itself, file transfer is at least twice as fast and even the internet seemed quicker when testing IE8 on a few pages (not that I intend to switch to that browser!).

I've still got more extensive fiddling to do on it and need to ensure it will run all of my programs and applications, that is coming up in part two, there are bound to be a few compatibility issues!