My little girls (4 and 2 years old) have got one of these, and they absolutely love it. The graphics are much better than I expected, and it already has a decent library of games. A lot of people (here and the leapfrog forums) seem to have had problems connecting it to their computer, but they work fine on my new PC and my 3yr old laptop. I suppose people are just far more likely to write reviews if things don't work than if they do. Just be warned it could take some time to set up, so don't leave it until the night before their birthday/christmas!
My eldest daughter loves the Princess and (especially) Dora games. They are challenging but fun and I'd say she's already got our money's worth out of them. She hasn't really touched Tangled, but that might change when she's seen the film. They got Toy Story and Mr Pencil for Christmas. Not much interest in Toy Story, but Mr Pencil is a HUGE hit! It's by far my 2 year old's favourite game and teaches fine motor control, aswell as numbers/letters. I can see this game helping her a lot as she starts school.
There are leaplets (apps) they can download from the leapfrog website. At the moment there are around 20 but I'm sure there will be more added soon. You get a code for a free leaplet with the system and they wanted "Jewel Train", a logic puzzle game. They got the hang of it straight away and it seems fun as well as educational. You can put each app you buy on 2 systems, so they work out even better value if you have two leapsters in the house. Also preloaded is a pet game. They have to wash, feed and play with her pet, and they can buy more supplies with tokens received for gameplay. It's great that there is plenty to do without even having to put a cartridge in.
This is a great toy for parents as well as kids. You can connect it to your computer and see what they've been learning, how many questions they've got right, and get suggestions for games that could help fill gaps in their progress. Some games have a skill selector to adjust the game to the level your child is at. Tangled, for example, has spelling lists you can create for them to play in the game. They have words for up to 12 year olds so it can grow with your child.
I can't comment on battery life yet as we are using a multivoltage adapter (leapfrog haven't released an official one), but from what I've read it can drain 4 AA's in a day. There should be a docking station released soon (it's out in the US), so that will be a godsend. One annoying thing I've found is that it turns itself off fairly quickly if left alone. That would be understandable on battery power but when plugged in you would think it would have a screensaver so they can start where they left off. She doesn't seem to mind though.
There is also a camera attachment which is good value for £15, the picture quality isn't great (as you'd expect really), but they have a lot of fun editing them.
Overall I'm very, very happy with the system, and I can only see it getting better as more games and leaplets are released. Well done Leapfrog!