The Xbox 360 pad is pretty great (NFGcontrols.com review) but it's not always the best solution. It's especially bad when you need an accurate d-pad.
Available in wired and wireless versions.
There are at least two 360 pad versions, and possibly more. The wireless one has a common ground and is very mod-friendly, but the wired pad has multiple grounds. This may not be consistent across all revisions however: forum member Waterbury says his wireless pad has multiple grounds.
There are two kinds of pots inside the 360 pad, two each for the analogue sticks and one each for the triggers.
All four analogue stick pots are the same, with a large range (approx 200Ω - 10kΩ) and movement (slightly greater than 90°).
The two trigger pots have very minimal movement (perhaps 45°) but the same range (0Ω - 10kΩ).
Both have linear resistance, and are 5kΩ at the half-way point.