This is with Yahoo's own mail app. Today one email took 30 minutes to show up on my phone. Same thing happened last night.
But it's intermittent, other emails show up right away. One of the problem emails was from a co-worker, so I sent myself an email from my work account to test whether it may be a server issue, but it showed up immediately, with a status bar notification and everything.
I checked the latest comments on the Yahoo mail app, and a few report similar problems. Otherwise, I'm not having slowness or force close issues, so I'll stick with it for a while longer, and hopefully they'll fix some of the bugs in a future update.
I was apparently one of the few who didn't have problems with Yahoo using the HTC mail widget (I still use it for a second, infrequently used yahoo acct.) I had it set to retrieve every 5 minutes, which isn't push, but close enough for my needs. The inability to check email over wifi was annoying though.