Fostex were making high quality equipment for the professional broadcast and film industry when the FM-3 was introduced. The FM-3 was aimed at the pro market too. In that sense, comparing equipment in the same market, Fostex and Sound Devices were in the same league but strictly speaking I don't think they competed in the same market for long if at all - Fostex were there with trolley based multi-track recorders whilst SD only had their 700 series portables. I don't think Fostex are in that market now?
Sound Devices appear to have their super-fans who are usually hobbiests and who dismiss anything else.. A company doesn't have to have a reputation or following to make great or one-off amazing product! So don't dismiss models one hasn't tried - try everything (if that's what you want to do).
I've just bought an FM-3 for personal sound recording, sound design, binaural recording - all for fun. Literally just got it and don't have any other pro gear to compare against for sound quality.. However, it is very well built as per other pro equipment I've been incontact with.
Searching around the internet for reviews/opinions on the FM-3's sound brings up hardly anything. Two opinions from those who've tried one: compared to a Sonosax sx-M32, someone thought there wasn't much between them soundwise for their preamps (although suggested the Sonosax was still better, so bought that) - http://rtsound.net/equipment/equipment/page/2/ . On a forum some guy simply says "they are excellent". That was all I could find!
Lots of critism for their small metering display though. At first glance, I think I'll find that to be correct. Display is too small for the style of meter they have chosen - too cluttered a graphic to be clear. But OLED was kind of new back then and a faster display than the LCDs being used on other gear..
Should I be able to compare it with something well known in the future, I'll find a forum to express my opinion for future generations!
New price was once about £1300 / $1800 (before Brexit GBP value falls). They can be found secondhand, looking new condition for about £300 in 2018 via ebay.