There are an awful lot of people with good things to say about Krashen, and an awful lot of bad things to say about him.
For a good, balanced summary of Krashen’s theories and some reactions, look here.
Krashen was/is an advocate of bilingual education. I hate almost everything about bilingual education, particularly past 2nd grade. And apparently some have taken that to a hatred for Krashen himself. Look here for a fierce anti-Krashen view.
So why my verdict? As with any SLA researcher, you have to take what makes sense and throw out the rest. And judge it by success. Does Krashen’s model match what your common sense tells you about how people learn language? It does mine. And as a teacher I believe I can see it happening. Besides, don’t be fooled into thinking these guys believe everything they write. Sometimes they write something super-strong just to have such an opinion. Last year I sat down to dinner with DeKeyser, someone who disagrees with almost everything Krashen writes, and some of my colleagues. He backpedaled so much it seemed he wasn’t even willing to espouse his own theory, not as he’d published it anyway. So take it all with a grain of salt, just the way we teachers do with everything someone hands us, right?