Basically, it was the play. They moved a few scenes and greatly condensed most of the ending, but it held up well. Lord Goring was charming, Mrs. Cheveley was slimy, and the other characters were quite good as well. I liked Miss Mabel, and unlike in the other version, they left her a funny, clever, and minor character. I hate when screenwriters feel the need to make minor female characters show up all over the story and take over (Arwen). But I digress.
I only had a couple of problems with it. There are spoilers in here, so go read the play. I've turned them a parchmenty yellow color, so you can highlight them and read if you want. First, the tremendous condensation of the ending left almost no time for the charming scenes between Lord Goring and Mabel. Her impatience to get Goring to propose to her, and her scolding of him, is so funny in the play, and it was largely left out. I suppose that's minor, but still...
The other problem is also pretty minor, but I find it annoying. Here's a bunch of spoilers. In the play, Lady Chiltern sends Goring a letter asking his help, which Mrs. Cheveley finds, and being a bad person herself, assumes meant she was having an affair with him. After Goring makes Mrs. Cheveley give him the other letter she has, she reveals she has pocketed Lady Chiltern's letter, and runs off. The ending is rather suspenseful because just when Sir Chiltern decides to stick to his guns, and Lord Goring has succeeded in keeping his past a secret, Mrs. Cheveley is scheming away to ruin the Chilterns' marriage again. In both movies, they have Mrs. Cheveley keep Lady Chiltern's letter a secret, and tell Lord Goring she has it after they all go to watch Sir Chiltern give his speech to Parliament (not in the play). While this is a more simple "good guys think they're safe but then the villain has one last trick" twist, I like the extra suspense of knowing her scheme the whole time, and hoping their success won't be ruined, and wondering how they'll get out of it. It works OK either way, but I like the play better. Also, showing the speech in Parliament is boring! It works fine in the play where it cuts to them back at home talking about what a great speech it was, while the audience knows the trouble that the letter could still bring.
Overall, though, it's a pretty good adaptation, and a pretty good movie.