I am a stay-at-home Mom who has planted my 2 kids in front of the TV whenever possible for a break. It's a mixed blessing, and I'd say TV's pros and cons depend on the temperment of you and your child.
My daughter has *always* been extremely needy, demanding and malcontented (since an infant). She also has never been a "good napper," and TV has always been how she unwinds and rests instead of nap time. (My son, on the other hand, has a happier disposition, naps well, and sometimes plays independently, so he's gotten a lot less TV.) Meanwhile, I am an introvert and also firmly committed to never spanking my kids (trying to "work with" my kids as often as possible to resolve problems rather than "doing things to" my kids to enforce behavior). The combination of 2 preschoolers, one of whom is a "difficult" child, my introversion, and my intensive parenting choices for dealing with "bad" behavior makes motherhood very draining for me. If I don't get some down time, I simply get very irritable and cannot function well with the children. TV helps a lot in this regard. When they watch TV is when I recharge my psychological battery, shower, clean the house, make necessary phone calls, etc.
Meanwhile, the kids pick up on a lot of good lessons from the quality programming (on TV and video--I only let them watch commercial-free tv). Some of the shows today are great, with problem solving, active female characters, good values being taught, etc. My daughter knows, for example, about rare animals and igloos and all sorts of other interesting things from TV that I have not had an opportunity to teach her in "real life." At age 5, she can also read a couple of dozen words and do some math (and knows all letters, letter sounds, numbers, etc.) because of the fantastic Leap Frog videos.
On the down side, my daughter demands a lot of TV and it's a fight constantly to limit it. She even wants TV when her friends come over to play, and will sit for hours if allowed, planted in front of the tube. My son really started picking up language once he became interested in TV, but he too is becoming increasingly couch-potato-ish.
What I have done is put the TV in the play room (and only the playroom has a TV) with all the toys. I don't think kids should have TVs in their rooms. I've also put uncomfortable chairs in the playroom (no couches for my couch-potatoes) to generally discourage family TV consumption. Often, the kids will play while they watch TV. When they ask for TV, I will sometimes make them look through the house first for a "more fun things to do." If we can't find anything together in their small fortune of toys that they'd rather do, I let them watch their shows. But, usually they come across some fun toy they'd otherwise forgotten existed and play for a while.
So, for us, TV is definitely a mixed blessing. I did want to say that the research shows a several-fold (if I recall correctly) correlation between TV and ADHD incidence. But be careful, because the overall risk of ADHD is very low to begin with -- increasing it many-fold still results in a very low overall incidence of ADHD. It's like drug companies touting that a certain drug reduced your risk of a rare disease by 500% -- say the risk of the disease is 0.005 percent to begin with. Now your risk would be 0.001 percent with the drug. Not such a big deal, even though 500% reductions sounds impressive.
Hope these thought and experiences with TV/no-TV help you decide.