You may wish to contact A'gon member, Plamenz. He is currently using the Uniks as rear speakers.
Yes, I have 3 Linn Uniks for surround speakers left, right and center. I am not going to tell you that hose are the best speakers, but I will give you the reasons why I decided to buy them:
1. I had already Linn front speakers, so I wanted a match.
2. I am moving to a new house where the builder will do the pre-wiring for the surround speakers using a gage 16 wire and no option to upgrade to a better cable. This for me pretty much eliminates the option to purchase good speakers for the back - no bi-wiring, no bi-amping.
3. The white Linn Unik with the white Linn Brakit will fit very well in the interior of my room (if you care about this kind of stuff).
4. It will be very expensive if you try to get all good speakers for 6.1 or 7.1 setup (at least for me it will be, since I have all Linn speakers setup), so now I have Linn Index in the front, Linn Trikan center. Linn Affekt sub and 3x Linn Unik.
For the price I think the Unik are worth it, but I would not compare them to my Index or Trikan in terms of quality. If you have a sub, the low freq should not be a problem plus the back channels are only for effects.
Of course the ideal situation will be to get all identical speakers. I would not recommend the Linn Unik for the front or the center, but for the back I think they are doing a good job.
Hope this helps!