I've been using Pentadactyl with bleeding edge Firefox since its first days and
I definitely agree with you.
Pentadactyl doesn't support Firefox 15+ so I had to downgrade to a more stable
channel with every major Firefox release. After I ended up in the stable release
with no Firefox 15+ support in sight I began looking for alternatives and to my
surprise Vimperator is still alive and kicking (3.5 got released six days ago)!
I don't get runtime errors anymore, it looks and performs much better (not sure
if I should attribute the performance increase to newer Firefox) and has way
better UX choices (like how minibuffer overlays the page instead of resizing it
with every keypress which has a performance impact with heavy webpages)
I suggest every Pentadactyl user who doesn't need the better vim compatibility
or extended configuration options to check Vimperator out, its still in
development and certainly didn't get abandoned or superseded by Pentadactyl
despite what people are suggesting.
I looked at Vimium but was put off by its implementation. It seems that Chrome simply doesn't allow a plugin to be as completely integrated as Firefox.