I've had no real trouble until now (running 7 x64 on an Intel E5200, 4GB DDR2, 6800 GT + FW 181.20), but it seems to have some visible minor issues:
Font scaling is somewhat screwed up in (both FF3 and IE8), with large portions of the same web pages displaying large fonts where there shouldn't be any, and small fonts where they should have been bigger, alternatively.
The second is somewhat more puzzling, but i think the mainboard (Asus P5B-Deluxe WiFi/AP, with the latest 1236 BIOS) and RAM configuration are the culprits.
You see, the 4GB are arranged in a 4x 1GB fashion, with one stick of each brand per channel (two pairs, one from Transcend, the other from Infineon, all at SPD timings).
Now, if i leave the "memory remap" feature enabled in the BIOS, all the native windows apps detect the 4GB, but then i have the occasional long slowdown/freeze, where even Ctrl+Alt+Del doesn't work for up to a full minute.
If i leave it disabled, then Win7 x64 will detect 4GB on the performance rating screen, but the system info (Start->Search: "System Information") will report 4GB of "Physical Memory" installed, but only 3GB of "Total Physical Memory", as does "Task Manager".
CPU-Z 1.49 detects the correct amount at 4GB. Running on Dual-Channel DDR2-800.
Windows Bug or BIOS limitation ? I know this motherboard supports up to 8GB, so, I'm at a loss about it.
Other than that, it's running beautifully so far.
The major difference ? No more hard drive "scratching", a constant hell of mine whether i run 2, 3, 4GB RAM systems, one or more fast hard drives, 32bit or 64bit versions of Vista, etc.