I play R:TW mods at 2560x1600 resolution and have no problems.
Some people complain that the icons are 'stretched' in widescreen mode but they look OK to me.
I don't play 'vanilla' R:TW.
This is what I see on the RSII campaign map (the drop down message icons on the left do look fuzzy to be honest):
I'll try and post what I see on the battlemap (I use the 'minimal UI' interface for that).
The 'minimal UI' can be set up using the preferences.txt file (change MINIMAL_UI:FALSE to MINIMAL_UI:TRUE) - if you do so I suggest that you set up the control keys (in the Options menus) for the battle speeds as these are the buttons that vanish between the stanndard and minimal UI interface (I set mine to Pause=Shift+P, Normal=Shift+N, Double=Shift+D and Triple=Shift+T).