Quickbooks Pro won't give you project management or diagraming like D-Tools, but for a couple hundreds bucks and a couple hours setup time it will give you excellent managment of customers, vendors, inventory, AP & AR, payroll, etc. Much of this can be done with the MS Office Suite, but I expect it would take days to weeks of time to reproduce even a portion of the functionality QuickBooks has?
Some other things that QuickBooks will do that you'd have a difficult time creating with Office apps is Merchant Services for accepting credit cards, Sales & Use Tax and a whole slew of business and financial reporting available with only a few clicks.
After working with large ERP/MRP apps like Oralce in the past, I was amazed at what a $200 piece of software can do!
Some other nice features are special tax export and reporting tools for tax time and also the fact that when you grow large enough to need some help with your books, it seems like almost every accounting type person has current or past experience with QuickBooks and knows how to use it. I've got someone coming in for a few hours each week to help out with this and it works really well. This would be difficult to do if you "home grow" your own system.
I know a number of companies with revenues in the millions that still run on QuickBooks. There is a point where you outgrow it, but if you supliment it with some of the MS Office apps for project managment (Project, Visio, PowerPoint, Excel), I think it will handle the financial side of the business for quite some time.
Cheers,
Paul