I think the Monzas definitely are undervalued, and it is quite hard to tell why. The chromed one is a bit much for me, but a 150.501 is, I think, a fantastic looking watch.

Maybe the lack of model variation/proliferation compared to just about every other series of Heuer "named" watch means that the appeal isn't there for the vertical collector, and even the avid horizontal collector is probably going to be happy with one example. That said, I am a strongly vertical collector myself (Carrera) but this is one of the few other models that has a really strong appeal for me (others being any Daytona and maybe an olive drab Autavia).

Would be interesting to find out what other people's favourite series are (I think there are some we could guess :) ) and how they arrived at a favourite, and also how some are more drawn to the horizontal, "one of each" approach. For me, I have always liked collecting things that had strong similarities but with some differences too and the Carrera scratches that itch very nicely for me - so much proliferation that it will be near impossible to say that the collection is ever finished too!