I'm definitely interested in an arduino/ raspberry pi custom solution sounds very cool to me. I took the 'easy' way out and use an x300 temp logger, some details in the other post. I just foil-taped my sensors to the copper (pex even in a couple cases) and wrapped with pipe insulation. Agree with everything RSI mentions. I think my temps are pretty spot on, maybe a tad low. There is no issue with lag as you mention. I see temps about 2 degrees cooler than my OWB display reads, but quite a bit of that (or all) is just heat loss from my pipe run.
I got my sensors on ebay, real cheap, they look like this: