I have to say that I don't love or hate them. I enjoyed that fact that they were available in the first few generations. I was always mad that I couldn't get certain ones without Nintendo Events (I hate that). The only one I got from a Nintendo event was Celebi for GSC. Some of them like Ho-oh and Suicune are very appealing style-wise; I think some just look cool and I like catching and using them.
However, I will say that I think Generation IV went a little overboard on the amount of legendaries they introduced. Some just seem kind of unecessary or pointless, especially the two blue brine shrimp they tried to pass off. I think that they also should have waited till the next games to release guys like Heatran. I just feel as though unleashing so many legendary Pokemon in one generation kind of waters down the idea of legends in the first place. What's to stop them from bringing it up a notch from the current 14 of DP up to 20 legendaries in the next generation? That's almost 8% of the entire regional dex for each region. Less so for a larger National Dex, sure, but the point is that if they keep pumping out legendaries with each generation containing more than the last, eventually there'll be more legends than normal Pokemon. I think "legendary" loses its meaning when applied to so many new creations. If something becomes so common in the games, how can they be legendary? I think it loses its meaning.
I like some legendaries, but I think they're starting to overdo it and should wind it down next generation and focus on better, new non-legendaries. And if they really want to focus on legendaries, they could try giving us the oppurtunity to actually catch Pokemon like Mew and Celebi in the game without having to travel around for game events all the time or get stupid [random] Tickets to get to [random] Island.
Bah. I wish I could trade my Celebi from Generation II into my Diamond, but that two aren't compatible. I can basically never get a Celebi in a legit fashion unless I want to move to Japan and stalk Nintendo there to see when the next Celebi is given away. That or get it through a trade, which I don't see ending well for me. Humbug.