Ok so we havent played this much yet but with the Lego games you do tend to get a good sense of what's on offer fairly early on. And it is pretty much a follow on from Harry Potter 1-4, which brings both good and bad
The 'good' is the sheer fun and attractiveness of the game, especially in two player. We played 1-4 as a family game and people could drop in and out at will with it all working seamlessly most of the time. That is true again (the lack of online co-op is shame but not a big deal, to us at least)
5-7 continues the story from the films as you might expect, but it adds more complexity along with a few more spells and abilities to make the game a little different to 1-4. It looks to have a bit more to explore too, which is great, since for us getting to 100% was easily as much fun and completing the story mode
The puzzles are still easy (it's almost like they have run out of ideas) but that does mean 100% is within the grasp of even relative novices (ie younger kids, with some grown up help) if they are determined and persistent. One frustration for us is getting stuck because a specific character is needed to perform a specific task - and it isnt always obvious who/what/why that is. The new combat thing seems a little out of character for the game but I guess we will get better with practice - given the focus of the later films does this too, I guess it make sense
The graphics are more 3 dimensional (there are time when characters become very small/distant on the screen) but can be quite glitchy but it hasnt spoiled the game for us yet. One huge improvement is the brightness control, 1-4 has so many dark sections it can be impossible to see/know what you are doing - no excuses now
Another slight dissapointment (for us at least) is having got 100% in 1-4 will give you no advantage at the start of 5-7. Some games let you bring some things/abilities with you, this one doesnt. And they have changed how spells work a little so you need to relearn your buttons (ie 1-4 dig is Wii button B in 5-7 it's button Z)
Ultimately, if you have liked playing 1-4 you are probably going to enjoy/want 5-7 to finish the story. If you enjoy 5-7 without having played 1-4 then you probably will want a copy of that too (though the price of 1-4 has gone up quite a bit since 5-7 came out so they obviously expect demand)
UPDATE - well after a few weeks playing the new spells and exploration are still fun but we are getting a little fed up with glitches. The structure of the game is such that it isnt always clear when you are in a story segment, but sometimes if you dont complete the story segment for some reason you get stuck later on and have to begin the whole game again. We got all the way to Just Desserts in Story Mode recently but didnt have the Focus spell to move on - so we had to start over from the beginning and hope whatever we didnt do last time we do do this time. This is proving rather less fun that HP 1-4, so much so the kids arent playing with 5-7 any more. I guess it was rushed out too soon to tie in with the film release and/or Christmas.