I have a pair of LSR 305s sitting on my monitor stands right now - sitting next to a pair of Sceptre S6s. It is an interesting pairing!
I really like the Sceptre S6s, and those will be my main near field monitors until I move into a purpose built (and larger) room. But I also like the LRS 305s.
It took me a couple of weeks to get used to the S6s, to understand what they were telling me. The stereo image is astounding, there are no oddities in the cross-over region, and they hide nothing.
The JBLs, on the other hand, sounded right on the first day. They don't reveal every problem, and the stereo image is not quite as detailed, but they are great sounding monitors. I believe I could mix on them and my mixes would translate well.
I don't really expect either the Presonus or JBL monitors to get better with time - I expect that I will learn more about what they are doing, and that will make them more useful to me. But I remain absolutely amazed at how easily I adapted to the JBLs. (I suppose that could have something to do with the fact that I got them second, but I don't know that for sure.)
If I can figure out a way, I'd like to keep both pairs - they are just different enough that I think the combination would be very useful. I will be keeping my UREI 809s<G>!