I've got one (Tecra M4) and I've tried two (Tecra M4 and RM OneTablet).
Having seen all the differences between Slate (RM) Vs. Convertable (Tecra) and Intel Integrated (RM) Vs. nVidia GeForce Go (Tecra) I'm going to say: Go with a Tecra M4.
Yes, it cost's a lot, but it's well worth it.
Sony Vaios that cost more than the Tecra come with a 1280x800 screen (96dpi), I've got a 1400x1200 behemoth @ 128dpi (has to be seen to be believed, and surprisingly, it doesn't strain your eyes)
The only real problem with the Tecra M4 (and Portege M200) is the rather weak single joint, but you can easily open it up and tighten the screws every 6 months or so (and without the Warranty Názis even noticing).
Since the Tecra uses the Wacom digitizer (I don't know about the RM One or some of the "lesser" tablets) you get all the functionality of a first-generation Wacom Qintiq display (pressure sensitivity and support for more advanced pens, woo!) which is great
for airbrushing in Photoshop (warning: requires another driver download, and I've had some problems with it not recognising my stylus' eraser)