I'm an editor by day (medical and scientific content and academic nonfiction) and a writer by night. I second what HarryT says. I can handle a few typos in a book (editors and proofreaders are only human after all), but repeated grammatical mistakes are annoying.
Grammar isn't easy. Even after 15 years in the biz, I still learn new grammatical concepts, and styles and language (even spelling) change, so it's not as if you can ever learn everything. I do give writers some leeway, but good grammar raises a writer's credibility immensely. The nuts and bolts of grammar and spelling are relatively easy to master even though they are often boring to learn about (there, I said it). Still, polished prose and beautiful writing are wonderful to behold, so it's worth taking the time occasionally to work on grammar a bit by picking up one of the short instructional books that are available.
At the moment, I'm certain that I'm better at editing than I am at writing fiction. But I'm enthusiastic about both.
