I have a 11.1 system (with Audyssey DSX Wides and Heights). My left and right fronts are Synchrony One towers, with a Synchrony One center. I wanted the additional speakers to be tonally matched with the One's up front, but thought spending the money on Synchrony One B's for effect speakers would be overkill. After careful thought, I chose Imagine B's for the Wides (very good match, as well as reasonable low frequency extension). I chose Imagine Mini's for the left and right surrounds (I prefer monopoles, and the Mini's have the same tweeter as the Imagine B's, so the sound field is quite uniform). IMO, the Heights are much less important, and the Rear Surrounds are the least important, so I am using small Definitive Technology bookshelf speakers that I already had for these two sets of speakers. Had I not already had the Def Tech's, I would have considered additional Imagine Mini's (which are incredible speakers for the price).
All of my speakers are crossed over at 80 Hz (100 Hz for the Def Tech's) to my subwoofers, so the slightly different low frequency extensions of the Imagine B's and Mini's is not a factor. However, the tonal similarity of the tweeters is extremely important.
Since rear surrounds get such little action, I think Synchrony One B's would be extreme overkill. If you like diffused sound, the Synchrony S is an excellent choice for side surrounds. If you want direct sound, the Imagine B's or Mini's are highly recommended.