I can't speak for the Fostex or R09, but I love the D50. IME the line stage and A/D are top notch, and the deck is very robust, both on a physical level and on a firmware level. Bottom line, works great, sounds great, very reliable.
I found mine used for a good price and just got it, without any need or plan for it. Figured for my occasional stealth or as a backup to my 722. I've since got rid of the 722 for a 702, and got rid of that for an Edirol R44 (Oade CMod). Still have the D50 though and use it all the time, generally using it instead of the OCM R44 unless I want 4ch. I'd say the line/AD stage stacks up to all those recorders. Lately I've been forgoing my V3(digi)>D50 route and instead running EAA PSP2(analog)>D50, very compact and sounds great.
Anyway, bigs thumbs up for me on the D50 -- my favorite recorder I've ever owned except for the SD 7xx boxes.