I'm of two minds about this. I'm prepping for another downloading spree. Right now I keep my game pretty much Maxis Match, as I do with all my Sims games. Default skins and eyes that are slightly better looking than EA's, but still pretty cartooney. Hairs that have similar textures to EA/store hairs. I don't download clothes because, frankly, there's plenty in there already.
But way back when I first started downloading CC for TS2 I always went for anything that looked vaguely good in any sense of the word and shoved it in a bloated downloads folder. There's something charming about playing The Sims with ultra high poly emo doll hairs and high res nipple overlays like everyone who ever made Sims movies did.
My question to you: do you go subtle? maxis-y? or all out super model/doll? Or just download anything and everything? Or nothing at all?

Anything and everything, although I do lean towards high poly realistic looking stuff.

I don't try to look for a specific style, or look specifically for Maxi-Match, so I suppose that means I download whatever I like. I tend to like things that look good but how realistic depends on what it is. I'm picky about the skin I use and some would call it realistic, but I've seen one or two more realistic ones that I'm not interested in at all because I don't like the look of them. I suppose they are a little too different and I don't think they fit the game. I feel the same way about some of the hairs out there.

Ultimately I think the whole concept of "realistic" or "cartoony" is subject to personal definitions. What's too cartoony to me might be too realistic for someone else.

The realistic looking stuff is a bit too Uncanny Valley for me, and I despise photo-textured clothes and hair where all the bits that should naturally hang off the body kind of stick out, so I stay away from a lot of custom content that seems to be popular. I guess that puts me more in the Maxis-match or cartoony camp?

It honestly depends on how good the CC looks. I usually go for the more realistic skin tones and eyes. But for Hair and clothes, I have a little bit of both. Like Indigo mentioned, sometimes the more realistic looking hair ends up looking...unrealistic on your sims (at least a lot of the longer hair styles do) so getting custom hair styles and hoping they look good in-game is like playing the lottery XD

Whenever you get CC hair for women, it's usually best if it's a short hairstyle or ones where none of it hang over the woman's chest. Otherwise you could end up with a bad case of gravity defying hair.

I always go for maxis-match with ts3. With ts2, I can simply retexture any hairs that I think look good, and then bin them. But I have no idea what I'm doing in 3, so I just stick to downloading maxis-match

I download just about anything that I get an "ooh that's great!" gut reaction to. I tend to focus on trendy hairstyles and "ethnic" hair because I have ridiculously curly hair and so should my self sim. I also like custom sliders and clothes/accessories that strike me as cool. For me familiarity breeds contempt, so I like to have a lot of options. The things that came with the base game are like visual nails on a chalk board for me. There are a very few things I can stand using and it's because either I feel their design is actually good or they are generic enough that I they fade into the background for me. I have downloaded a few custom skins, because I found the musculature of the default skins to be odd and I really wanted my sims to have nipples and navels.

For me everything has to match one style. I like the looks of some of the other hairs and things but I'd fling my computer if I saw random made sims in a different style so it's just easier on my nerves to simply stick with the maxis style. I never really had a problem with the original look either and think it looks pretty nice too. That being said it's really up to you and what you like. It's your game so mix and match or be as consistent as you want. Just keep in mind, the more cc you have the more it's gonna slow down on loading.

Anything, so long as it looks good

To be honest, CaST reduces the need to choose because you can always recolor and pattern things to be as detailed or as stylized as you want.