I've got a Mitac Mio pocket pc with Tom Tom installed, which works pretty good.It has its' own aerial built in & is otherwise the same as a normal pc.You can also use it to install some scan tool software on if you want.I think the reliability of getting you to a place mainly depends on user operation,ie entering places correctly etc, but if you can get a post code for where you're going, it's pretty foolproof
Hope this helps, Regards, Andrew