Black Dog said:
When I raced as a junoir we were limited to a 50 tooth maximum on the front and we all ran 50-39 or 50-42. No point unless you are running a compact with a 34. There will be a lot of duplication between the 50 and the 39.
I wouldn't be so sure about the "a lot of duplication". I just did a quick calculation of the ratios (too lazy to convert to inches ...), which shows the following:
(Excuse the formatting - shows up fine when I type or edit, but goes to hell when I post it ... Just move the 50, the 39 and the top two ratios to the right one column and all is good)
50 39
23 1.696
21 1.857
19 2.632 2.053
18 2.778 2.167
17 2.941 2.294
16 3.125 2.438
15 3.333 2.600
14 3.571 2.786
13 3.846
12 4.167
(Blanks are 'cos I know that no-one uses those crossed up chain lines ...

)
To the OP - I run a 46-39 on my cross bike, with a 12-25 on the back. I have also run it with a 12-23 on the back (and a 53 on the front, but that's another story ...) Out of cross season, the bike gets used as my commuter - which means anything from 15 to 35 km road rides each way. I find that it works fine. Sure, there are times when I curse the 46, but it still lets me get along at a good enough pace. And of course if you're in the mood for a LSD ride in the small ring, there's no difference from a standard roadie.
I'd have to say that I'd rather have that set up to a compact crankset. My bike originally came with a 46-36 "cross specific crankset" and I definitely prefer the 39 as the small ring - for racing and general riding.