Yes it is, most have and will still play unmodded..
One thing however mods are not the issue with modding, its the utilities that having loads of mods requires you to use, if you start modding my advice is to make sure you get these first..
BOSS, better sorting of mods, WRYE BASH ( the managing of mods ) and OBSE ( a script extender ) you will need these, and the Construction set.
Learning to use them and making sure you are familiar with placing files in the correct folders is the biggest issue I found, but once settled it becomes very easy imo.
Mods however do add a lot to the game, and some are just as good if not better in production due to focusing on fair sized aspects of the game.
OOO ( Oscuro's overhaul ) or FCOM when combined with other overhauls is one, as is better cities ( a beautiful mod )..
Race balancing project and L.A.M.E add a lot and do not require much for their experience needed for moding.
Edit: Ninja'd it would seem