I can't speak to the MR8, but I have the MR5's and they are awesome. If you compare the Mackies to the competetors in the price range, you will see that they supply more power to the high's and lows. The Mackie puts a whopping 100W to the low and 50W to the high, where the KRK Rockit's only have 70W to low and 20W for the high. I'm of the philosophy that the more power you supply the less the speaker has to drive at a reasonable db level.
Some people say the Mackies are too bass heavy, but I don't think so, however my one complaint would be that they don't put a -5/-10db PAD on the low freq, but they do put a PAD/Boost on the high end freq, so you can compensate for this. This is one of those debates that's like which came first the chicken or the egg. If your speakers favor too much bass, do you under represent it in your mix, or if your speakers don't have enough bass, do you over represent it in your mix? Again just be aware of it.
I looked at the Mackies the KRK's the M-Audio's, and I feel the Makies are the best deal of all of those.
