I don't think I've designed a single electronics project that I really consider to be "done". At times, I think the constant re-working of designs is unhelpful, esp when there are a few people who are very keen to see a finished product.
The thing is, every time I go back to looking at how the design of something is, I find another iteration that improves the capabilities, makes it easier to build, or makes it less expensive.
Take for example the BBPSU. It's now on revision 3 or 4 (I forget actually, "2.0" is not technically true) and there are still likely to be more revisions. It's not that it's a bad product, but it can be improved on.
For example, I've used PTH DC jacks with large round holes to solder them to for a long time. DC jacks have slots, but slots can be difficult depending on the board house and a load of other factors. The fallback for them is large holes. They're a pain to solder.
Similarly, using a P-Channel FET for the reverse polarity protection is a neat idea, but as the footprint I chose is a little tight it can cause production problems. A diode isn't as compact but it's got a lot more tolerance in it and at the current ranges of the BBPSU, it's not that bad.
The regs were SOT-223 for compactness, but it's become more obvious that it's not the best package for the 5V regulator - which leaks a lot of heat. More recent boards have used a D-PAK version of the same regulator, which sinks heat much better.
As I said, there's nothing actually wrong with the BBPSU, it works and does what it says on the tin. As you can see from the rambling above, it also doesn't mean it's beyond being improved.
The challenge I now find myself in is picking the right time to call it "done enough" and produce some devices for people to use, before going for another iteration. That's still something I'm working on.