just got through compiling & testing one of our bread & butter apps against apache flex 4.9.0. compiled ok first time, passed all our base tests A-OK and while we still need to run benchmarks its looking "just fine" as far as user interactivity goes. production file sizes are more or less the same, debug versions run slightly larger than adobe 4.6 SDK (in our case 90kb+ bigger).
downside is that the design view in flash builder won't work w/this version (but then again flash builder 4.7 doesn't even have a design view any longer). one workaround is to layout using the 4.6 SDK then switch to 4.9 to compile/debug.