Any suggestions on keys/key combos that are good candidates for custom mappings etc?
[snip]
I think Bram mentioned he's found prefixing with _ works well..
I believe that the backslash ("\") is the only stand-alone lower-ASCII character available for mappings, and thus is the default map-leader (":help mapleader"). Depending on your vim environment, control+S and control+Q are also available, but on console versions, they ofter mess with your flow-control so they're not quite as portable.
There are a family of characters which mostly duplicate behavior of other keys, and thus are good candidates if you want them. Such characters include the underscore (as you mentioned), the plus, the minus, the <enter> key and control+M/control+J. These characters mostly navigate up or down with tweaks regarding how they treat counts, and how they behave column-wise. (Always a bit like my understanding of the kama-sutra..."#58 is the same as #42, only you cross your fingers")
Fortunately, Alt+<anything> should be available if supported by your vim environment (some consoles don't pass along Alt). Additionally, all the <F-n> keys are available (again, assuming availability in your terminal).
Some folks have problems with the backslash on their keyboard because it's in an inconvenient place (or requires key-chords or use of dead-keys), so they prefer to use another character as the mapleader. However, if backslash is available and easy enough for you to reach, it would be my recommended character.
A lot of folks seem to use the comma or semi-colon as a map-leader, but they _do_ have functionality that I use on a regular basis. Therefore, if you don't want to relearn muscle memory and want to eventually learn to use those commands (a good follow-on lesson to using t/T/f/F for jumping in a line), I wouldn't go over to the dark side... ;-)
-tim
