Painfully bad, I love it.
Badly executed, with impossibly wooden dialogue and acting so stiff you'll think you paused the movie. There is no mystery in Anakin's turning, and knowing how cool Darth Vader is we don't pity him. Anakin has had over 6 hours to make us like him and yet my only impression is that of a whiny man-child. An argument can be made in his favour: his upbringing in the Jedi Order, whose religion is the amalgamation of male stoicism. This matter isn't approached, as expected from big-strong-man-loving Hollywood.
Padme's role is nearly non-existent, and her finale is lazily written. It's clear Lucas is trying to aggressively shove all characters to their New Hope starting positions.
The Anakin vs. Obi-Wan climax is a masterpiece of music and choreography, and surprisingly effective in story-telling. The one virtue of this dumpster fire.
All that said, such a wealth of memes have been created from the horrible dialogue, and it holds a special place in my heart and many others' despite being pathetically bad at all it tries to be.
I recommend it only if you're willing to watch 7 seasons of Clone Wars beforehand and fall into the prequel memes fandom.