Personally, I love the WM9; fantastic watch with lots of building potential. I owned one of the early V1's, purchased directly from WM9 and modded it with gen crystal, bezel complete, crown, etc. However, the economics of such a project may be something you want to keep in mind if considering such a build. Depending on how far you go, the boundaries between franken build and pre-owned gen may become quite close. For that reason, if you are really considering a WM9 and want to add things like a gen crystal ($250 +/-), gen bezel ($400 +/-), gen dial ($300 +/-), gen bracelet ($750 +/-)... Maybe take it to the next level with a gen 3135 ($1800 +/-) to go with the gen dial and a set of gen hands... Well, you're either at the cost threshold of a used 16610 or possibly even over that figure depending on how much you pay for the individual parts and basis watch.
Don't get me wrong- If you stick with the minimum of mods, it's possible to stay within 25% - 30% and still have a great watch! I suppose that's the key with such a piece; there are certainly areas where the watch would benefit upgrades and the sort, but you have to be willing to accept other aspects of the watch as-is. A lot of that depends on your level of expectation and how particular you are when it comes to such things. Because the 16610 is not a terribly expensive gen pre-owned, I often suggest the gen to anyone that seriously wants a 16610; the reps can get very close, but will never be 100%. With the cost to build one out... The math is easy