I don't mind on a personal level, if the US government wants to waste money looking through my internet history it's their problem.
On a theoretical and general level though, I think it's profoundly dangerous. I just don't think governments should have the ability to spy on individual citizens - yeah, the current US security apparatus with its current priorities doesn't particularly care about me, but do I trust that there will never be a US administration that might ever want to interfere with anything I could conceivably do? Because I don't think that's the case at all. Privacy of communications is a fairly integral right as far as I'm concerned, it's really important for the maintenance of political and religious freedoms. People have a right to oppose actions by their government or another that they disagree with, and they have a right to express those views without those governments being able to snoop on them and thereby take action against them. Whilst terrorist groups of the sort we have at the moment I clearly oppose, the US has been known to prop up pretty nasty dictatorships and regimes in the past and if they're into large-scale international communications tapping there are worries there too.
Clearly where the security services believe there's a real threat, they're going to need to look at communications, but that's what judicial warrants are for. Pre-emptively scanning and tapping communications or doing so without a warrant opens up fundamental issues in terms of personal freedoms, because even if the current security services are hunting for terrorists who I vehemently oppose, if they can read everyone's communications then supposing a government wanted to start opening files on their political opponents (McCarthyism style) they could just roll ahead and do it.
