I'm taking Scerri for 20A, and honestly the course is not hard. Scerri is pretty entertaining (I've only fallen asleep during lecture once) and he teaches well. The English accent is a plus. Additionally, he PODCASTS his lectures, which is the best thing since sliced bread. However, you should not ask "stupid" questions, because he likes to make fun of them (and the person who asked them).

If you took AP Chem and AP Physics, the first half of the course will be almost review. I really like how the course is mostly conceptual. You are not required to be able to derive any equations. He gets his exams questions from previous exams, so make sure to study from the back of the course reader, which has a bunch of old exams. To be honest, our midterm was nearly an exact copy of a previous midterm.

His exams are pretty fair, but make sure to read the questions carefully. Sometimes he changes variables around or phrases something differently. So if you don't pay attention you end up solving for something that is related, but still wrong.

Written homework is pretty easy, however, each TA will grade by their own standard. Pay attention to detail and vocabulary, if possible, just copy down exactly what is written in your notes/course reader. Thinkwell quizzes are annoying because some of the questions are on topics that Scerri did not really go over, requiring you to spend 10 minutes on Google to find out whether or not, say, EM fields are quantized.

Rumor has it, a decade ago, Scerri wrote and published the rough copy of a 20A course reader. Every year students point out hundreds of errors in the course reader, of which Scerri corrects about two. Doing so is just enough to merit releasing a new version, thus discouraging students from buying used copies. Perhaps, by the time our children are in college, these corrections will have been made, but I wouldn't bet on it.

On a more related note, I would recommend this teacher. His extreme arrogance is quite entertaining, especially when he tries to incorporate his personal ideas into the course curriculum. This is most likely the easiest non-GE you will take at UCLA and requires little more than reciting his course reader. Since there is only one midterm and the first five lectures are spent going over the plum pudding model and what a valence electron is, don't even bother showing up until at least 3rd week if you have taken chemistry before (or you can read wikipedia). On the other hand, it would be a shame to miss any of the numerous times Scerri shoots down overzealous freshmen.

Scerri is a funny guy and a fair teacher. I just hate chemistry so much. Got a C+ in the class but that's my own fault for sleeping in class and not taking the class seriously. Still a pretty cool guy though, great sense of humor but a pretty damn large ego. I would advise taking another teacher though because Scerri teaches Chem 20A in a very conceptual way that doesn't quite prepare you for computational heavy Chem 20B taught by most other professors at UCLA. If you can get Scerri for both Chem 20A and 20B then by all means, take it. It'd probably be easier than the other professors. But if you're a chem/biochem major, perhaps you'd be better off in the long run with another professor.

I had Scerri for CHEM 20A and I thought he was great.

Lecture: Like everyone has said, he teaches everything conceptually, which he is very very good at. He concentrates on ideas instead of math. I liked that he covered a wide range of topics. He is required to teach you quantum mechanics, but he expands on other stuff, like isomerism, crystal field theory, and a little bit of organic chem. He exposes you to a lot of different things.

Exams: Honestly, if you memorize the course reader, exams won't be hard. Everything in the course reader is fair game, no outside stuff. Don't be afraid to question things on the exams like typos and stuff. They say he writes his exams the night before, so be careful if something looks funny.

Course Reader: This was the only thing I had beef with. He charges you roughly $65 for the course reader. He makes the font obnoxiously large so there are a lot of pages, so he can charge a lot. Seriously, it could be condensed into a single volume.

He will diss you in front of the whole class if you say something stupid. Don't screw around. If you can get past his personality, you'll do just fine.

I would recommend.

Professor Scerri is pretty good. You only need to buy the course reader, NOT a $150 textbook or his book about the periodic table, so that's a plus.

He's a good lecturer, truly understands what he's taking about and explains well. His tests are about understanding the material. Just pay attention in class, read the course reader, and go to discussion and you'll be fine.

A downside is his personality. Very arrogant and insults you if you ask a dumb question (sad for you, but hilarious for the rest of the class). Just ask your TA questions.

The best thing about this class: it's relatively easy. All you need to do is study the course reader (you need to study it REALLY well, though) and you'll be fine for the tests. The thinkwell quizzes are annoying but they are pretty helpful. His other homeworks aren't that hard. Also, he has past exams included in the course reader, a lot of them, which are really really helpful. A lot of them are repeated too.
The worst thing about this class: He's not that good at teaching and doesn't know what he's talking about sometimes. During the first half of the class, with the quantum mechanics, he teaches it really confusingly and with a lot of holes and freaks the students out. You don't really need to learn much QM for the tests though - just the main points, and you can find these by looking at past exams. He's pretty arrogant and so is not that concerned or available, but he does still let the students know what they need to know for the tests. Basically, this class is pretty easy if you study the course reader back to front.

Had him for 20A. He is a little arrogant and mixed to many topics in an introductory course. The course reader he used is kind of disorganized, which makes it not very effective to review at the end of the quarter. Learnt the second part of the course almost by myself.

Coolest professor I ever had! If you have to take 20A for your engineering preparation or GE fulfillment, believe me, he's your best choice!

Scerri explains material clearly and makes the class not so boring with his jokes.

Just a warning: this class is very easy, but the curve is VERY HIGH. A small error on your test can bring your grade down to a B due to the extraordinarily high curve.