Mildly. It's easy for me to spawn a new tab or window when I want one, and when I don't, I expect the link to use the current one, so having _blank set makes me take an extra step. But there are so many sites out there with crappy design that limits my control of my own browsing in more serious ways (resizing windows, flash sites you can't link to, blah blah blah) that it's a pretty minor annoyance in the grand scheme of things.