Well, Trillian doesnt interface that well with MSN, and MSN has some really cool features, and operates smoothly. I like it, just give it a try, and see what you like and dislike about it.
The GUI... after using only MSN up to 6.1, then seeing what Yahoo and AIM are like - the GUI on them are crap! I could write programs to look visually like Yahoo or AIM; but to write one to look visually like MSN would take a good deal of time. Let alone then with the features.