I've been using it since it came out. My opinion is that while it's not perfect it's better than manually checking the mobile yahoo email site via the browser.
My experience has been that it doesn't seem to affect the battery too much, if at all. Not noticed much of a difference.
The 'major' issue I have is that something when it moves between wi-fi and G/3G/H it seems to get stuck and the push mail doesn't work. If I kill off the task then restart it then it works again. The other issue is that occassionally mail can get stuck in the outbox and there's no manual force send option. The 'refresh' menu option on the main screen sometimes works, but not always.
I don't think you can turn notifications on with mobile data off. The two options you have, it seems to me, are:
1. leave mobile data on all the time and have notifications on, so you always get mail. (One thing it does do well is if you've got it set like this, go into aeroplane mode, then later back turn off aeroplane, it will 'bring down' all the mail available at that point. That seems to work consistently IMO.)
2. have notifications turned off and when you turn mobile data on you manually go in the app and tell it to refresh.
I think the first option may do what you're after, but IME it just seems best to keep mobile data on all the time. Battery drain isn't that bad.