I just received my MBL 121 speakers this week and am very happy with them. As mentioned above though, they need oodles of power, which I do not have just now. I will be upgrading amplifier shortly though. I have a key candidate in mind, but I'd prefer to keep my choice under wraps until I receive it (hopefully used, if/when one comes up).
I heard the full MBL 101E system at THE show in LA at the end of '06 and was blown away. I couldn't agree less with Stringreen - these speakers breathe so much life. My friend and I were literally laughing at how good the sound was - effortless, open, everything I could want. It was one of the three best rooms in the show for me, if not the best.
I like the idea of adding a sub or two later. This is likely not easily done, having experimented with subs in previous years. But I suspect two Fathom F112s or 113s could work very well, perhaps with an outboard crossover like the Bryston. I intend to try that eventually.
Good luck with the 121s if you get them - they are a treat and so beautiful, but like all aspects of MBL - beauty is in the eye/ear of the beholder. Happy listening