My Xenon Xbox360 had begun randomly freezing and/or red ringing. It is far out of warranty so I figured I'd try fixing it myself - why not? Worst case, it would be fun to take apart.
The directions on iFixit were incredibly detailed, I was very impressed. Was happy to have the plastic spudger - I didn't really use the metal ones I'd picked up, but I used the plastic one a LOT.
I decided to also reflow the solder (in addition to installing the RROD Fix Kit), so I bought a heat gun too.
The most important thing seems to be taking plenty of time to clean off the old thermal paste. Mine was hardened into a plastic-like substance around the chips and very thin and gooey on top - it seemed doubtful that it was being effective at wicking away the heat. It took many, many scrubs with Q-tips and rubbing alcohol to get it all off. Chip off what you can around the chips, delicately, and then use the pointy tip of the spudger in combination with the Q-tips to slowly pull off the rest. It took quite a bit longer than I expected, but I knew if I didn't try to do a perfect job, and the machine still red ringed, I'd wonder if I needed to open it all back up and try again.
It's unclear to me if I needed the heat gun or not - I reflowed the solder as described in another guide, and that seemed to go well. The Fix Kit plus the heat gun are still way cheaper than buying a new Xbox though, so while you're mucking about inside your Xbox, why not reflow?