I used the GA-K8N Ultra-9 in a system I built in June:
Gigabyte GA-K8N Ultra-9
AMD Athlon 64 3200+ Retail
Corsair 2x512 MB Value Select PC3200
MSI GeForce 6600 GT 128 MB
Sound Blaster Live! Value
Samsung 120gb 7200rpm 8mb cache SATA hd
Black Lite-On 52x32x52x16x combo drive painted silver
Silver NEC 3520A 16x DVD+RW/-RW
v.92 56k PCI modem
Enermax 485W PSU
Antec Super Lanboy case
The motherboard booted right up with an Athlon 64 3200+ Venice, despite having the old F2 BIOS, which lacks support for Venice cores. I flashed it to BIOS F3 anyway.
I'd been a bit weary of the passive heatsink on the board so I ran 45 minutes of memtest86 as soon as I turned the computer on for the first time (after configuring the BIOS, of course). The heatsink was very hot and I could barely hold my finger on it. So, I voided the warranty and installed the updated Asus hsf from my A8N-E--I replaced the hsf on my A8N-E with a Zalman NB-47J and no longer needed it. The new Asus hsf felt cooler to the touch. However, it understandably added some noise.
Since I've built so many computers, I didn't bother reading the manual at first. However, this was the first Gigabyte board I've used. A HIGHLY important note: Press Ctrl+F1 when in the BIOS to access the "advanced settings". By "advanced", they mean memory timings, clock speed, and voltage options. Prior to reading the manual, I couldn't believe that Gigabyte had omitted such essential BIOS options. My bad...
Also, the temperature and voltage monitoring features on the GA-K8N Ultra-9 are designed for very basic users. From within the BIOS, the +12V, +5V, +3.3V, and CPU core voltages are reported merely as "OK" (or "NOT OK" if there's a problem, I'd assume). In Windows, I'm baffled by the voltages after trying several system monitoring programs, including Gigabyte's own software, which agreed with Everest. None of the sensors are very accurate and there's one sensor that's stuck at 71C (sometimes 70C). I've noticed that everyone with these boards has noticed the same odd sensor. Ignore it. The fan rpm sensors are correct, and the GPU and hard drive sensors reported believable numbers, but those are unrelated to the motherboard.
I ran Prime95's Torture Test for over 24 hours straight on the GA-K8N Ultra-9 followed by 7 straight hours of the computer hosting and viewing a 32 bot UT2k4 match. It passed both tests without trouble.
Other than the inaccurate voltage and CPU/motherboard sensors, the board presented no problems. It's been running perfectly for 6-7 weeks now.
When I built my own socket 939 system two weeks after assembling the GA-K8N Ultra-9, I used an Asus A8N-E 2.0. They're both solid motherboards, but I prefer the A8N-E's layout. The PCI-E 16x slot is located higher up on the board for better airflow and the nF4 chip has more room, which allowed me to install the Zalman ZM-NB47J.
A few weeks ago I contacted Gigabyte, Asus, and MSI regarding the nF4 thermal specifications. MSI wasn't helpful, but Asus and Gigabyte informed me that the chip can handle "0-90C".
So, Gigabyte's passive nF4 cooler is probably safe according to nVidia's specs. The NB47J on my A8N-E feels much cooler than the GA-K8N's heatsink did though, which is probably due to its larger surface area that catches some of the case airflow--the Gigabyte cooler is too short to receive much fresh air.
I haven't OC'ed either board, but if you want to OC, I recommend avoiding both the A8N-E and the GA-K8N Ultra-9. I've heard too many reports of BIOS bugs when OC'ing the A8N-E, and the GA-K8N Ultra-9 doesn't have adequate cooling for higher speeds.
Bottom Line: As long as you don't plan on OC'ing, you'll run the computer in normal ambient temps in a case with decent airflow, you don't require maximum GPU cooling, and you don't care about voltage, motherboard, or CPU software readings, the GA-K8N Ultra-9 should suit you well.
Edit: I forgot one thing since it didn't affect me. Notice how tightly packed the memory slots are. A few people have commented that RAM modules with heatsinks barely fit and can't get much airflow. The slots on my A8N-E are spaced out and don't present that problem.