I used to own a pair of RP8G2’s a few years ago, and now I have a pair of RP5G2’s as secondary speakers. I like both of them, but there are a number of caveats.
First of all, the RP8G2’s are incredibly bassy, and this is greatly augmented when you place them on resonant surfaces (like a desk). When you have them on proper speaker stands away from the wall, it is not so bad and you could actually mix on them if you take the time to get to know them. But they are still bass-heavy and you have to allow for that when making mix decisions, which is -of course- not ideal. When placed on a desk and/or near a wall, they are only good for non-critical listening. But they are great for parties, as they go loud and sound good for most modern music.
I like the RP5G2’s much better. When used as nearfields they have enough low end for most styles except for really bass-heavy styles like Hiphop/R&B or EDM. They are also not as sensitive to placement as the 8’s. Downside of the smaller woofers is that they start to choke when pushed, they don’t go very loud. But you could get a reasonable mix balance on these, provided you listen at moderate levels and have something else to check the lowest octave or so with.
I would not call either of them neutral sounding. They actually sound a little polished and they are somewhat short on detail. My biggest beef with them (more so with the 8’s than the 5’s) is that they are a little fatiguing, especially over prolonged periods of time and louder levels. The 5’s do not go that loud so this is not as much of a problem with them, but it is definitely a part of the series’ character.
I somehow wonder whether the RP6G2 would be the sweet spot of the series? The 8’s are too bassy, the 5’s are too light and don’t go loud enough…
But with all that said, I think they are as good as it gets at their respective price points. The obvious competitors are the Yamaha HS and M-Audio BX series. Between these series it is really just a matter of taste as they are all competent stuff for the money. I went with the KRK’s because they sound a little more hi-fi. The Yamahas have a more forward “monitor” sound but lack depth. the M-Audio BX8 sounds similar to the KRK RP8G2 whereas the BX5 is -oddly- closer to the Yamaha HS50.
If you are looking for something on a budget, they are all good choices, so check them out (preferably in your own room) and get whatever works best for you. I do recommend to upgrade to better (but pricier) speakers as soon as you can afford it, though. These speakers do the job, but none of them will give you enough detail.
edit (September 24, 2011):
Thanks all for the comments. I kind of want to backtrack on the “fatiguing” statement. It was definitely an issue for me on the 8’s, but I have used the 5’s on an almost daily basis over the last few weeks and it hasn’t been a problem.