My 4-year old loves ABCmouse. She loves to sing along with the various alphabet songs, she knows the character names, and she particularly loves talking about how many tickets she has. She has definitely really leapt ahead in her letter recognition and general understanding of the alphabet since she started playing.
We did try the personalized assessment (which costs extra) and found it was not really worth it -- it didn't actually change anything in the base app experience which is where our daughter spends all her time. But the base app is pretty fantastic.