As we've said on TBD and elsewhere, choosing a concealed carry firearm is all about compromises. As MB said, the derringer may be a great second gun, but it wouldn't be any of our choices (I think...) as a primary if we are really trying to be prepared for a worst case scenario.
In the last 4 weeks, I have carried on 2 days. I have spent most of my time in Europe, where it wasn't a practical/legal option. I am writing this now from Hollywood, where it is not a legal option for me. So, for me, ANY derringer would've been better than what I had (knives, pens, creativity, etc).
So the real question isn't "is this gun good enough?" because we can't predict what you'll actually need. I think the real question is "how much compromise is enough?"
Some people ask that question, especially from a keyboard, and then rant about how their safety and their family's safety is worth never flying on a plane, never going to Manhattan and generally never being without 2 guns, a knife, a cell phone and a chem light. I don't feel that way. Obviously, I compromise my ability to protect myself and those I am with very frequently. As long as you realize that you are doing so when you choose to carry the derringer instead of something else, you are making an educated choice.
Now, all that said, for me, if I am going to go to the effort to carry a gun, I generally want it to be a fair amount of gun. The hoops you have to jump through to carry a 5 or 6 shot revolver instead of the derringer are only a small percentage increase... one that I think is worth it most of the time.
-RJP
**edited to add***:
as for what to go to after the derringer, a lot of that will depend on your body type/shape. I carry my j-frame appendix carry frequently with only an untucked t-shirt or button-up and jeans.... but there are plenty of flatter solutions that should work in a variety of areas. At the end of the day, it may come down to compromising your dress habits for the sake of gun.. which may or may not be worth it to you.