My listening room is of moderate size, and I don't really have an acceptable way to mount speakers on the sides of the room, so my surround speakers are located behind the listener. After experimenting with both direct radiator, dipole, and bipole speakers, I eventually settled on a compromise that works very well in my room: M&K SS150 THX tripole speakers.
The M&K SS150's have drivers on both sides of their angled cabinets, so they act like dipoles, but they also have a somewhat larger driver on the front panel, which provides direct sound. The speaker also has provisions so you can adjust the relative balance between the side drivers and the front driver, thereby changing the distribution pattern of the sound. I have found this speaker to be nearly ideal for my needs, and it is sufficiently flexible that it can be used in almost any placement configuration.
The SS150's aren't cheap (MSRP is $1200), but M&K now has several similar models that are a good deal less costly. I was able to find a used pair of SS150's for $500 here on Audiogon, and at this price they were an excellent value.
For more info about M&K's full line of surround speakers, go to their Web site at the following link:
As an aside, the Snell surrounds are excellent also. Snell has a tripole similar to the M&K SS150, which I seriously considered (I think the MSRP is $1500/pair). I chose the SS150's mainly because I was able to get a good buy for a used pair, but I also thought they offered a bit more flexibility in my listening room.