Aschenbrenner is a decent teacher as far as the Math department is concerned (Math major here). His homework and lectures are reasonably related to his exam material. On top of that, he's a pretty likeable guy, always making odd jokes here and there. I generally find that it's hard to have a bad math professor who is under the age of 35 (which our boy Asch is).
If you're a fairly competent math student, this professor'll be alright for you.

Aschenbrenner is a really nice guy, and he was awkwardly funny at times. He can't teach though, in my opinion. If you are good at learning on your own then it shouldn't be a problem. The homework for this class is never that long and his exams were very fair, save for one question on midterm 2 which was practically impossible (one TA called it a "cheap shot") and not even related to the homework he gave. But the class is curved anyway so it doesn't matter. The final was not overly hard and you get plenty of time on it. Overall I would recommend him since he isn't hard (though not easy either) and is a pretty cool guy.

Ascenbrenner is a really nice guy. The first day of the class, he tells you that the class is curved, and that scares the hell out of you because you know only a certain number of people will be getting A's and some people will definitely fail. But as you move along, it's not all that bad. He does a lot of proofs to try and get you to understand the math so you don't have to memorize any formulas. His tests were not really all that bad. I remember getting a question on the second midterm completely wrong because I flipped a minus sign to a plus sign, and I still got 9/10 points. Overall in the class, I had a test average of about 87% or 88%, and about 90% on the homework, and I ended up getting an A-. You'll end up learning a lot from him, and he's not really a tough professor at all. I do recommend him, even though I heard Park was easier. (P.S., the curve ALWAYS helps you)

Professor Aschenbrenner is not really the best professor I've ever had. He's always showing up to class disheveled and it's very easy to get distracted by a variety of his colloquialisms. This having been said, his lectures themselves are straight-forward, and his homework is not particularly difficult. It is strange, then, that his midterms and final were so difficult to perform well in. The grading is very strict and there are almost always some sly tricks which make it very easy for students to mess up. He's very strict with partial credit as well so make sure you check your work many times over. My quarter there were two very different curves for the same class, so I would make sure you try your best to get the best TA in your lecture.
He's a pretty good professor though.

Professor Aschenbrenner is a great person but not so much a great professor. His lectures are very straightforward and follow the textbook almost word-for-word, which is not the greatest motivation for you to attend lectures. After all, you can just read the textbook. However, his lectures are not too bad. He is a genuinely nice guy, and he seems to care very much about his students doing well in his class, and I guess that makes going to lectures just a tad bit more attractive. His tests (two midterms and a final) were all very straightforward. He taught us everything we needed to know to do well on his tests. He even gave us hints one day as to what would be on the final. Both of the lectures he taught had very high averages, which in a sense is good but will destroy your grade if you don't score close to or higher than the average.