I have been running Quickbooks Pro for a couple of years on a Mac using Parallels. Except for the problems I have had with Parallels since moving to a 2010 MBP, it has worked very well. It is a little bit of a pain since I don't like to leave Parallels running all the time, but it has worked.
However, since having some quirky Parallels problems (Hard Disk 1 error), I have decided to go with Quickbooks for Mac, since they finally have a Payroll option, even though it is different than Enhanced or Assisted. I don't understand why Intuit can't just make a cross-platform program, as I truly believe a TON of people would start using it, because if it wasn't for Quickbooks, I would have completely scrapped the Windows virus years ago.
I have now almost completely moved my office (8-10 computers) to Mac and haven't looked back. Office for Mac is finally coming up to par and Mail/AddressBook, iCal, etc beats Outlook hands down, IMHO, at least with our POP3/IMAP office. MobileMe also rocks for live syncing to the iPhones and administrative assistants.
Quickbooks is the final switch that will get us mostly running Mac (except for the secretary needing to run Parallels for stamps.com software, which is only for Windows). I don't really like the feel of Quickbooks for Mac as much as the Windows software yet, but I can leave it open all the time and it works, which is worth a lot for me. I can deal with the Payroll functions being online and with the small differences in the Merchant Services, and we don't really use it enough to truly need multi-user, so I'm fine. For you, however, I would go with Parallels, maybe in Crystal mode, as was mentioned. If you keep the windows installation clean, it should run really well and fast for you.
I assume I'm not the only one who hates the Windows update system, with a completely new OS to learn every several years, with every other one being a complete flop and waste of time (ME and Vista). With Mac, for the last 10 years, I guess, the new OS upgrade has been MOTS with new features that are always cool, and the Snow Leopard upgrade has been the best of all.