I'm not normally one for heartwarming family movies, but David Tennant and Billy Connolly were enough to make me put this on in the background...and then I became enthralled and enjoyed myself thoroughly. Gratefully so, because how the movie actually played out didn't match the description.
Billy Connolly is great, as usually, and is the breath of fresh humor that works so well against how uptight everyone else is. He's immediately the grandpa everyone wishes they had. Irascible, humorous, and definitely has stopped giving a fuck.
David Tennant is great, of course, and I'm not just saying that because you always remember your first doctor. He plays the overwhelmed and distant so well.
The stars of the movie are the kids, though. They're so great, and the writers gave them so much to work with. Utterly believable, and they earned so much sympathy from me.
Yes, it IS heartwarming, as the trailer says, but there is so much more to the story. It's about loss and coming together and family and it just needs to be seen to understand. There's drama and humor and sorrow, and it is a perfect combination that isn't normally achieved.
I'd watch it again, and I certainly think you should watch it too.