From time to time you may read that Intuit needlessly dropped QIF import from Quicken. Intuit has stated that QIF was dropped because it wasn’t robust enough. I won’t speak to the strengths or weaknesses of QIF as a format, but I can tell you that Quicken 2006’s support for QIF is lame.
Others have documented that, despite Quicken’s claims to the contrary, you can import QIF files, so long as they have a "!Account" header added. This is true – to a limited extent.
I’ve got a Quicken file with over 22,000 transactions in it. Pretty much my entire financial history for about 30 years. (At one point, I went back and entered data from before Quicken existed. I had waaay too much time on my hands.) Because I had reason to suspect that there was some corruption in the data file that super-validating wasn’t fixing, I decided to "clean up" my data by exporting the entire file to QIF, creating a new Quicken data file, and importing the QIF back in.
After exporting, I checked the QIF file, and it looked good. After importing the first time, the results were terrible. One of the problems I had was incomplete import of split transactions, and another was that import had somehow renamed all my utility bills to "My Bank Home Mortgage". So I created a new file, and turned off transaction renaming, and I imported my accounts with many "split" transactions first.
This worked better, for some small value of better. Instead of being off by a million dollars over 30 years, I was off by only $100,000.
I spent some time comparing the before-export and the after-import files, and comparing them to the QIF file. When the QIF file has a transaction with many split items, and some of the later splits are transfers to other accounts, they aren’t imported.