The only way to really know, with anything SEO related or otherwise, is to do it yourself. Most of what people tell you here are half truths. Test it for yourself. Set up multiple domains, one with just one topic, one with all the subdomains. Check your stats often.
I'd say if you don't overdo it, they will hold nearly as much weight as a separate domain, especially if you can get a shit ton of links in to the main domain
I've always gone with separate domains though, it's just my gut feeling that they will get a better nudge up the serps doing it that way.
