This is absolutely fantastic! I've only properly played the extra levels so far, which are fiendishly designed, and loving every moment of it.
It's brilliant that each mode (original/extra/mixed) has it's own music - and especially that the extra levels have the "Technician Ted" music!
This is my favourite kind of remake - it has the original game but then builds on it with new content and features. If you enjoy 8-bit platformers then I can't recommend this highly enough.