I was introduced to the Lego games by watching my sister play. They looked fun, so once she'd finished Harry Potter Years 1-4 I borrowed it. It was fun, aside from me discovering that if you use Snape to open the Slytherin common room door you can never get into the Slytherin common room and therefore never reach 100%. So I effectively completed the whole game twice. The end is quite fun because you play as Voldermort and just have to get a certain number of studs, which I did mostly by driving the Knight Bus into things.
As my sister was taking a while to get through her other Lego games, I bought Lego Star Wars (as she only has that for the PS2). It being the first one they did, it's definitely not as good - Lego Harry Potter showed you what you could shoot if you waved the remote around, whereas the remote does nothing in Lego Star Wars. Also, there's something trying to kill you in every single level. And one level per episode in a ship, which I hate because it's impossible to steer.
But despite all that I have finished all the levels once (some more than once) - it's designed so you have to do them at least three times if you want to get to 100%. Not that I will because the bonuses at the end of each episode are to do all the levels in that episode straight after each other. As some of them take an hour to get through I tend to play for about an hour at the time I am not doing that.
There are extras you can get to make things easier and the more (useful) ones I get the more fun it is. Or in other words, I no longer die when people kill me, which makes it a whole lot easier.
What makes the games good in general is that they are slightly silly. The cut scenes re-enact relevant scenes from the films, just sillier. And there are silly extras, such as turning all your wands into carrots in Lego Harry Potter (they still work the same, they just look like you're wandering around carrying a giant carrot).
Mirrored from my blog.