The reason I'm not broke is that I don't use Quickbooks for invoicing. There are several good solutions for online invoicing. I use Harvest and I think they're great. No P.O.s, though.
The thing that sold Quickbooks to me is the existence of lots of Dummies-style docs -- you are hardly alone in your pain when you're a Quickbooks user -- and the fact that my accountant accepts the files, which is the real killer feature.
I'm still stuck in Gnucash for my personal finances, but I didn't consider using that for the business. I have little faith in my accountant's ability to make sense of Gnucash's files, I'm not confident that it has the features I need, and I can't seem to get it running easily on the Mac. The damned thing seems to be linked against every single library in Linux.
it doesn't do purchase orders because i've never had to use them, but i may be able to integrate what you need if it's not too complex.