I had those side by side at home for a while, actually, R300, R500, Q900, Song Towers and ST monitor version each auditioned for 2-4 months. R series and Salk are very different to say the least and both will find their fans.
I ended up with R500s although I think the Q900 is the most accurate from all the ones I listed.
I think that full and big sound on Rs are a bit of a lie (probably done on purpose to make them sound bigger) and I wouldn't call them detailed; very clean, tight, but not detailed as STs. But than again STs sound like their main goal is to provide the most details (toned down bass, forward midgrange and highs). So the real accuracy is probably somewhere in the middle.
My favorite part in the Rs is the tweeter, it just so delicate and realistic. With their snappy bass the overall result is very impressive. When I ask my guests to describe what they hear in Rs the first thing many say is that they play very clean.
To me the R series is kinda difficult to describe, I saw 1 review where the guy goes like "bass is a bit loose and this is wrong and that" and in the end he says that the R700s are the best overall speakers he had in his living room. I kinda think the same they are not perfect, but they just sound so good.
I agree that R series just puts many other speakers for the same price to shame.
I would like to hear the R900s as I think they might be better balanced than 500s, but I'm afraid the upgradeditis might strike if I hear those:)
p.s. There is a notable difference between the R300s and R500s, so the comparison should be R500s vs STs (same price), not R300s. Or something like R700s vs STs with RAAL would me even more appropriate. Don't mean to say bad things about stock tweeter on STs, it's probably good for the money, but KEFs Rs tweeter is just on another level.