If you have looked for a vintage Parker 51 aerometric in good condition recently, you probably have a bad case of sticker shock.
I have never owned or used a 51 (sadly) but decided to spring for the next best thing: a Hero 100. Produced in the old Parker pen factory in Shanghai, China, the Hero 100 is a fairly accurate tribute to the venerated 51 complete with 14K gold nib. The price? $40 shipped (eBay).
I have used my 100 for about a week and, after a bit of nib tuning and smoothing, I have deemed it good enough to be a daily writer. It is a big step up from the Hero 616 and is much more solidly built. Well worth the money, in my opinion!
More info on the Hero 100:
Pentrace Comparison of Hero 100 and Parker 51
Hero Pen Company