- Tae Kim’s guide has almost no exercises. Genki’s workbook has lots and lots of exercises that can be done outside classroom. Exercises take time so Genki’s slow pace is actually a plus.
- Genki has a slow pace but if you just want a quick reference to a grammar point, then instead of using Tae Kim’s guide, you are better served by using a Grammar Dictionary.
- Genki has lots of dialogues and other reading materials (in the second half of the book) so you are exposed to more context.
- Tae Kim has some weird personal view about the が particle.
- Lack of credentials. I never saw anyone mentioning this before so this is probably a minority opinion. But I tend to trust professionals rather than amateurs that think they know better:
Tae Kim started learning Japanese in college and ended the first year very confused and only able to use the polite form. Eventually, he figured out how to talk like a real person and started a website for learning Japanese (www.guidetojapanese.org) to explain everything he had to figure out on his own.
Since then, he has worked at a big Japanese company in Japan as a Java developer, passed the JLPT level 1, got a perfect score in TOEIC (the Japanese company administered it every year) , and continues to work on his next book for learning Japanese in his spare time.
I think the lure of Tae Kim’s guide is because one can read and finish it very fast but that grammar is not going to sink unless you do lots and lots of exercises but then you are better served with Genki.