Long story short, it was good. Go see it.

Long story cut in half and made into a movie? It's definitely worth watching. The script sticks much more closely to the original than the previous movies, and the liberties which were taken have largely been smoothed out. There's a bit of awkward, rushed exposition in the first few scenes (Oh hi Harry here's Bill and Fleur they're gonna get hitched oh and he got bit by a werewolf he likes his steaks raw and also Remus and Tonks are married and she is MASSIVE INTERRUPTION), but there are relatively few bumps throughout.

The Burrow, despite its earlier troubles, is apparently fully functional again. Which makes one wonder why exactly it had to suffer such troubles, but there you go.

The acting of the supporting characters is, of course, wonderful, and it's nice to see continued improvement in the abilities of Radcliffe, Grint, and Watson (Ms. Watson, I know you don't read this, but if you by some chance do: congratulations on the reduction of erratic eyebrow spasms. There's still some work to be done, though; keep it up!). Particularly of note is the supporting role of Yaxley, played by Johnpaul Castrianni, who does a fantastic job at acting the Gestapo-esque villain needed by Voldemort at the Ministry.

Plotwise, there isn't terribly much I can give away without making this a spoiler-tastic review. There are a few instances where they deviate from the book--they show some actions that Hermione takes over the summer instead of having her tell Harry & Ron about them, for instance--but they generally are done much better in the new style. There is more of a Hermione/Harry subtext than in the books, which is rather odd given Harry's obvious feelings for Ginny. Harry's somewhat maudlin nature does of course show up, but it generally emerges only in situations whose context calls for it. Additionally, not a great deal of time is given to the period between the wedding and the trio leaving Grimmauld Place, which I maintain is the best writing Rowling has done to day.

The most awkward difference I can probably think of is that the Taboo is not discussed, and so people suddenly switch from "Voldemort" to "You-Know-Who" without any reason given.

The special effects are the best they have been, obviously, and the various duels are a treat to watch, as is Ron's destruction of the Horcrux. Part One ends with the escape from Malfoy Manor and Dobby's death, which makes me hope for a Part Two that consists mostly of action.