Well, there is a lot to be said in favour of watching TV: 1) It is access to native material. Produced by speakers of English (or whatever language), for speakers of English. 2) It is 'natural' in that there is a context set already. 3) It is often more interesting than textbooks and so on. How you watch it would depend on your level. If you laugh at the humour already, then it would suggest you have a high level of English because you understand the jokes they make. You should record the shows, and listen for unfamiliar bits of vocabulary. Try work out by inference, and then look up the word in the dictionary. Note it down, try use it in a sentence. Watch the show again. With TV, sure, you can't learn just by passively watching, but if you concentrate, if you listen, if you repeat, it is definitely useful.

Well, SNL isn't exactly an hour of continuous laugh-out-loud material, it's often a hit and miss, so, being too distracted by your own laughing might be less of a problem than you think. I like it, but there are a few recurring characters who make up words or distort them, so be ready for that. It's also good to keep a basic idea of what's going on in the USA I disagree with Jerry on the filthy language, as they don't even use the F-word.

I suppose that it depends on your level. One thing is for sure, you'll hear a lot of slang, idioms and filthy language. If that's what you're after, go for it. They don't call TV, especially sitcoms, a vast wasteland for nothing.