To be exact, there are only three variables involved.

1. Targeted traffic
2. Website that converts
3. Product (great contents, ebooks, software etc)

If you have a lot of traffic which is directed to a badly design website and low quality product, you'll fail.

If you have no traffic but a good website but bad product, you'll fail.

If you have good product but no traffic and a website that does not convert, you'll fail.

But if you have reasonable amount of targeted traffic which leads to your website that sales your good quality product, your success is GUARANTEED!

I am not really a fan of mini sites although I tried the technique before. But I know mini sites really work for some people. I read someone who makes USD40K monthly from Google Ad sense alone from mini sites. But having one source of income is very risky because all those money can disappear overnight if Google makes changes to their ranking algorithms.

To ensure long term success, I create websites and offer a lot of content/ own products even though they require a lot of work to maintain. Such websites naturally encourage people to link to them which increases their PR over time. Next time when Google dance, those websites will have more chance riding the storm out and survive. Even if they drop in ranking, there'll still get many traffic from the websites linking to them.

That said, I stilll try to create mini sites once in a while if I find good topics, diversification is always good right.

If you notice your website income dropped recently, may be it didn't make it during the Jagger update (which is still running now).

For more info you can check out Matt Cutts blog, a Google Engineer here:

http://www.mattcutts.com/blog/

According to Jerry West a SEO, most heavily purged appeared to be those sites buying run of site linking (which Jerry has been warning about for six months) and those people stuffing / hiding text in their CSS style sheets and comment tags.