I have not used Quicken; I've used GnuCash exclusively. It feels a bit rough with the UI:
- Double-entry book keeping is a bit awkward; you wind up entering values twice, but the point of that is to reduce errors
- Budgeting is a relatively new feature, and I haven't looked into it yet
- You have to think of transfers among accounts, not categories or other fuzzy organization
- It's been around a long time, and the UI isn't flashy, but functionality is what really matters
- I suspect an Accounting 101 text would help me use GnuCash more effectively.
Balancing that, the data is stored in a gzip-compressed xml file.
The compression is also optional, so you can save it as a plain xml file.
This means that you have some hope of recovery if you wind up with a corrupted file. (And for programmer-types, you could keep it in source control for additional peace of mind.)
My wife and I have been using it for several years now, and has worked well for us.
LWN.net had a pair of Grumpy Editor reviews on personal finance software here and here which would be worth reading.