I used to reply in the comments if it was something that needed replying too but I just started having them come to my inbox so I can reply properly. I rarely go back to places I commented to see if there are responses so I figure no one else does either. I don't reply everytime though. I wouldn't want everyone to reply to my comments, too much email.
If someone leaves a comment on one of my blogs, I try to go back to their blog (if it is listed) and leave a comment to them. I have never emailed anyone back though. I did just receive an email from the owner of a blog I commented on this morning, and I thought it was nice to hear from her. Honestly, this isn't an issue I deal with much since I rarely get comments anyway!
I would definitely comment back even if it was just one line. I often comment my heart out on people's blogs and don't get a response and that bothers me. I appreciate those of you who DO comment back or at least visit the sites of those that comment. I think being a blogger means you should really share and spread the love around and it will come back to you in the end.
I reply to all my comment...I first reply via the comment itself and if I want to say something more then I follow up with a personal email. I think it's important to reply because the person took time to visit and comment. Not alot of people do, they just visit.
As for leaving comments I try to always leave a comment when visiting a site but sometimes it's not possible. With all the anti-spam setting sometime you have to jump thru too many hoops just to say hello, I read your blog. I have a baby in hand and limited time so for those people that need me to sign in, read and write a word and all that extra... I just I just read and keep surfing.