While it improves on the first, in the end this game feels like an unneeded visitor from the past.
But in the end, you really can't go home again. Sonic 4 Episode 1 was unable to live up to the lofty expectations fans had for it. It was too short, to easy, the physics felt off and most of the levels just rehased past games and some felt that perhaps the Sonic they once knew was gone forever.
And then something interesting happened. Sonic Colors came out and it was good. So good that everyone quickly forgot about Sonic 4 and shifted focus to this new game that felt like what a modern Sonic game should be. Then Sonic Generations came out and it was amazing and suddenly Sonic was good again. So enamored we became with 2 solid console outings that when Episode II showed up we were like "oh yeah, they planned to make more of these."
Sonic 4 Episode II does attempt to improve on a lot of Episode 1's issues and manages to do so but at this point its best to lower your expectations. Storywise it's attempting to connect to Sonic CD for some reason in what is already the flimsy Sonic canon. I had no idea what the story was and really don't care.
The major edition to this is Tails but this time around he's actually useful where he and Sonic team up for tag team moves. They can roll into a powerful ball to smash through things and Tails and carry Sonic through water and in the air. Better yet when you go to the new halfpipe Special Stages, you don't have to worry about Tails screwing stuff up for you, however these additions also make the game somewhat easy. Only the last two special stages will be significantly difficult but the ability to restart if you mess up also saves the hassel of repeating stages to get to the special stage.
As for the levels, most are still rehashes. Sylviania Castle is a combination of Aquatic Ruin and Marble Garden, Oil Desert is Oil Ocean with crappier music. It even rehashes the Sky Chase and Wing Fortress zones of Sonic 2. Winter Park should be noted though for taking one of Sonic 2's lost levels "Winter Zone" and finally giving us a finished product. I would've like to have seen Sonic 4 use more of those lost levels like Dust Hill, Genocide City, Wood Zone and Rock Zone. Luckily the boss fights aren't rehashes and you get to fight Metal Sonic. However they are still rather easy.
Graphics have been improved from the first game as well. There are nice little touches like fireflies in Sylviana Castle Act III and the hand painted clouds in the Sky Chase but sadly the same can't be said for the music. Honestly this is some of the worst music the series has had since "Sonic and the Secret Rings" especially Oil Desert Act II. Though Winter Park Act II does through in a bit of Twinkle Park's theme. Seriously where's the Sonic Colors composer when you need them.
A few other added extras are here too. The inclusion of Tails allows for multiplayer and the red rings first introduced in Sonic Colors are in this as well. Adding a bit more longevity to the game.
In the end, Episode II does improve a lot of episode 1's issues but now, 2 years later, it feels like an unwanted visitor from the past. You'll have fun with it but soon find yourself returning to Generations or Colors and with games like New Super Mario Bros. Wii and Rayman Origins proving you can sell a 2D sidescroller as a full disc, it's easy to see why Sega as of right now has no plans to continue with future episodes.