Like GoatMonkey, I used MicroFlat and did not find it to be particularly flat. It seemed to be somehwere between a satin and a semi-gloss. On the bright side, I found that I could mix it with ground pastel chalk to make a pretty decent mud for armored vehicles.
I tried Testors Gloss and DullCoat lacquers but wasn't totally happy with either. I found it difficult to to lay down a smooth gloss coat. The dull coat had a tendency to have batch to batch variation in flatness, sometimes flat and sometimes satin.
I currently use Future for gloss coats and Future/Tamiya X-21 Flat Base(4 parts Future to 1 part Flat Base) and have been quite happy with the results. I haven't used it long enough to speak from experience about any yellowing effects, but from the concensus of what I have read on the subject, I am not worried about it.
Andy