I have a C-C phase 2 remap (and all the necessary hardware) and regularly see MPG in the low 30's back & forth to work, which is mostly on B roads.
On a longer run (100+ miles on M4 & M5) to Somerset I can average around 34mpg, keeping with the flow of traffic (70-80mph).
Yesterday I went to pick the missus up from work and got stuck in a slow but steady flow of traffic... according to the trip computer I averaged 38.7mpg over 42 miles! Should have taken a photo really!
However, on a good bit of 'driving road' you can easily see low teens
As people have already said, it depends how you drive...
By the way, I had a remap (won't mention which) when the car was only a few months old, and it ran like a bag of spanners - when it ran, that is. I was given a list of excuses & things that could have failed causing the running issues, but apparently, it definately wasn't the software.

Anyway, I wasn't happy, and I had the remap removed. The car ran absolutely fine after that.
After almost 2 years of 'shall I, shan't I...' (having been put off remaps!) I decided to go for C-C phase 2. After all, I spent a fortune on the hardware & it would be a waste not to have the software to get the best out of it all. My first thought was "I should have done this 2 years ago!"
The car's now 2 1/2 years old and 30,000 miles... had C-C phase 2 for about 6 months... I've had no issues or replacement parts at all. Still on the original MAF, coil packs, N... thingy wotsit valve...
(you watch, now that I've said that everything will break next time I start her up! lol)