I don't upgrade all the time, but save them for (usually) Thursday or Friday afternoon rush hour flights out of NYC, where being apart from the madding crowd is really worth it (and these flights are pretty much always full and sometimes get ground delays).

Of course, part of what's changed my math on this is the disgustingly large displacement of mainline service in/out of STL by RJs, on all carriers (except WN, which is irrelevant to me). Since I've been forced to get used to sitting in ERJ seats and also to work hard never to end up on the awful CRJ-200, mainline coach doesn't seem nearly so bad to me, especially if it's in E+.