I worked at one company that used them (after a domain was disabled for high bounce rate). We found that most of the invalid emails were from transcription errors, typically typos in yahoo.com, gmail.com etc. Most don't catch the none@email.com type of errors, although frankly those are easy enough to validate yourself. My general feeling is that if you're collecting emails electronically you don't need it, but if the email list is created via someone doing data entry for paper records, then they're helpful.