I've used Samsung Omnia and HTC Touch HD with different shells on Windows Mobile 6.1, and I can't say I liked the system a lot. Aside from being sort of slow and clunky, you had next to no freedom of customization and usually had to hack your way around that. I also found that Windows Mobile applications rarely worked right and crashed half of the time, so I was kind of stuck with a system that looked highly customizable and useful at face value but was really just kind of slow and worthless.
A bit off-topic, but the Nokia n900 is looking like a sweet Linux-based open source alternative.
http://www.youtube.com/watch?v=JKOWlgXN26s#t=2m00s (the review is in swedish)