....how realistic are they?

Well I know from my brother in law who works for the federal gov in law enforcement that most of the really high tech "toys' they use on these shows are well out of the budgetary reach of state and local law enforcement and even the feds don't have all that stuff..... so they rely on the old fashion stuff, like fingerprint and bullet analysis and some DNA in the higher profile cases. Tax money only goes so far. I also know that these shows can be a bit of a headache for courts cause jurors are expecting the kinds of scientific analysis shown on these shows and wonder why police haven't use this stuff.