I use QuickBooks for my business.  It’s not a large business, really one person at the moment.  I played with earlier version of GnuCash but I can’t remember exactly why it didn’t suit me.

+ QuickBooks also allows automated downloading from banks like Quicken but I haven’t configured it like that.
+ Multi-currency support (it took years for them to implement this!)
+ Well accepted, my tax accountant knows it well and can ask me for information and tell me how to get it to her
+ Support via forums is ok

– This is not inexpensive software, I paid several hundred dollars for this and I update it every few years (like every 4-5 years).
– Even though I bought the software, they still charge for support (better to use their forums)
– They put out a new update each year which you have to pay to upgrade to, very few new features, mostly just a UI work over each year.
– They implement new features at a glacial pace.  It took them like 10 years to add the multi-currency support to the US version where the other versions (like UK and Canadian) had it.
– Probably the biggest negative for me is that QuickBooks is not really suited for a web hosting business or any business where you track customers with a monthly (or some periodic) payment like rental income.  It’s more suited for businesses selling parts.

A friend of mine suggested FreshBooks.  I have not yet tried it though.  It does not look like a replacement for QuickBooks, it seems like it would handle only the client billing part, something would still have to enter the bill into FreshBooks.