Let's get the main thing out of the way - Key's a moe-embracing company. So if you're one of those anime fans that think that moe is the spawn of Satan and is killing the industry, avoid their products at all costs.
I've never been a big fan of authors that "brand" themselves - i.e., everything Anne Rice does involves vampires, and all Christie did was mysteries. Key was made into the mammoth it is today primarily by one man - Jun Maeda, who has certainly branded himself - lots of moe; depressing, melodramatic plot elements; and bittersweet endings. Oh, and crappy, un-erotic H scenes, but hardly anyone cares about that.
The great thing about Key's works, though - and the reason why I'm a such a huge fan - is that when they excel, they excel hard
. The ending to Makoto's saga in Kanon was the most depressing thing I've seen in a visual novel, bought me to tears, and let me recognize the ability of VNs to be an artistic, literary medium, and greatly made up for all the (frankly, quite boring) slice-of-life stuff beforehand.
So, if you're interested in Key's output or the visual novel game genre in general: ignore the copious amounts of moe, get past the manga-style illustrations, and bear with the not-so-sexy sex scenes, and you might just find something magical, something moving, something heart-wrenching. At least, I know that I did.