Out of interest, do you have a good way to compare two parts of the same file using vimdiff without creating two files? Say a huge xml file with two nearly identical <parts></parts>?
As someone who moved from perforce to VS/TFS2010 at work, the first thing everyone at the office demanded back was p4merge. It's a very good tool.
Fortunately, with VS2012 the built in merge-tools has become much better and allows you to do inline editing in the basic compare/merge-view as well. Since VS2012, I can't say I don't miss p4merge, but I miss it a lot less.
If you are into this stuff, Araxis Merge is also worthy of consideration. It does a better job of char-by-char diffs than p4merge, and will recalculate the diffs after you manually edit the result.
The (old-ish) version I used had only 3 panes, though, making p4merge superior for 3-way merges. p4merge was easier to use from the keyboard, too. But for ordinary merges, I much preferred Araxis.
I've been happy with BeyondCompare. Reasonably priced, does 3-way merges, configurable with many version control systems. Now with Linux & Windows versions (but not a OS X version yet)