DD1 (nearly 6) watched far too much cbeebies, I would park her in front of it for hours, she was zombified to it, so it made my life a lot easier. She started watching around the 2 year mark, but it became obsessive, and she would have serious tantrums about wanting it on. Being a coward (and absolutely shattered), I gave in and let her watch it.
She is now being assessed at school, as she has a language disorder, mild learning difficulties and autistic traits... the language disorder was identified first, and I believed that this was because I wasn't engaging her in conversation enough and letting the tv do the work (upon reflection, its very likely her speech/listening would be behind whether I had let her watch hours of tv or not). But anyways, I panicked and stopped her watching as much tv. She didn't respond very well at first, but I persevered and she gave in.
Now she isn't particularly fussed about tv like cbeebies, so doesn't watch any at all. We do let her watch a dvd or 2 at the weekend though, and I think she enjoys that more than being hypnotised by repetitive tv shows all day.
As with everything though, its about moderation...