I have used the R5 (borrowed from a friend),and currently own and use the R8.
It is really a matter of how much money you want to spend.
R5 and R8 deliver the same pictures with the same lens placed in front.
Both have quite good metering systems.
R4s and earlier are rather basic stuff, with some technical quircks.
After R5 nothing much happened before the R8.
Likewise, the differences between R8 and R9 are neglible.
Picture quality is breathtaking. - some of the lenses seems to outperform the M series in practical work.
Focusing is manual and slower than the M series.
You need the mindset of the manual camera owner.
If you are into autofocus, the machine-gun F5 from Nikon could be the thing. The Nikon optics are not near as good though.
Then again, if I could carry only one camera for travelling, it would most likely be an M.
The best source I have found for R-series info is
www.summilux.net
Svein.