Pros
Good benefits, mostly good work/life balance (in software, at least), ESPP is nice (though the stock has been terrible).
Cons
A coworker was verbally abused by our manager. After the coworker brought the case to HR (as per illumina's own training), HR squelched the case and exonerated the manager. And once the dust settled, the manager quietly moved to retaliate against the coworker, giving him/her tedious work unrelated to software development, and eventually putting him/her on a performance improvement plan (PIP). Of course, this had nothing to do with performance, but rather with personal animosity for daring to expose the manager's bad behavior. HR decided to do nothing, and even reprimanded the individual for bringing it to their attention. Taking no action in such a situation is bad---it's worse if you profess certain values about a non-hostile work environment and don't take them seriously. illumina eventually gave the manager a promotion.