I have used three kinds of flex track, so the best way to review them is to do a comparison. This is for code 83 track.
Appearance - Micro Engineering is the most realistic. Then Walthers, then Atlas. Walthers is closer to Atlas but slightly better.
Ease of use - Walthers/Atlas, Micro Engineering. Walthers and Atlas flex track actually flexes; by that I mean you can easily bend it and it will return to its original position. ME flex track, OTOH, does not flex at all. It can be bent, but it stays in that position. I do not care for this behavior at all.
Reliability - Walthers/Atlas, ME. I have never had a problem with Walthers/Atlas track, however the ME track, because it is much finer, has issues with the rail coming free of the ties. This can cause serious problems if you don't catch it when laying your track.
Price - Atlas, ME, Walthers.
So in summary if you absolutely have to have the enhanced appearance of ME, go for it, but be careful. Otherwise if you don't mind the additional expense get Walthers. If cost is your main concern, get Atlas - it's fine.