I've been using the 2005 and 2006 versions (both regular and network edition in 2006) in my office with few problems, but ever since upgrading 2 weeks ago to the 2007 version, we've been having nothing but problems: sales defaulting to CC payments regardless of what payment type is changed to, inventory problems, even upgrading was problematic and took me (the person who usually get things to work right the first time) a good chunk of the afternoon just to get the primary workstation updated. There is no way to downgrade the data file back to the 2006 version, or I'd be using it again.
That said, MYOB is the _only_ accounting program that handles credit card transactions on the Mac (QuickBooks only does it on the PC version, MacPOS can do sales but not accounting and with the $600 extra just to accept credit cards becomes less attractive).
Yes, over the network, it can be slow, so whatever computer is doing the most work with it should be the host computer.
Unless you are well versed in accounting and keep impeccable records (entering everything right away), you should probably be sure to get support, since if you need it you don't want to shell out $$$.