My opinion of the place - amazing location, but pretty bad food, expensive drinks, and somewhat weird entertainment after sunset. Absolutely go there - but do it right.
Here's how you do Cafe del Mar - show up about half hour to an hour before sunset so you get a very nice seat by the wall. Have a drink or two, but no more since the drinks are ungodly expensive. As in, only foreigners can afford this place. To give you an idea - a beer from the mini bar from my really nice hotel or actually any nice restaurant in the walled city is around 4,000 COP, maybe 5,000 COP. This place charges 10,000 COP.
Definitely don't eat the food - it's sub-par and again, ungodly expensive. Enjoy the sunset and the nice breeze. Take lots of pictures. Take in the music, they play trance music which I think goes well with the sunset. Leave once the band starts playing.
The band they had one of the nights we stayed much past the sunset was sooo weird. The singer took great pride in his language knowledge and sang songs in English, Spanish, and French. He butchered the English songs, and I'm sure the French songs.
' soooooooomeeeeeeer tiiiiime and the leeeeeeee-ving is eeeeeesieeee' yeah. Awkward.
Oh and when he started to sing Bob Marley with his Spanish accent, I almost lost it. Like laughing.