I keep a spreadsheet to track all my online work (I use Open Office, which is an open source product that is free to download)
The spreadsheet is organised with one sheet per niche, with each page detailing all the properties in that niche, and a master page that pulls in info each month from the sheets to help me see which niche is profitable.
Well the surprise is that the "most profitable" niche changes from month to month!
I hadn't expected that. I had thought that I'd hit on one, maybe two niches that were profitable, which I could then concentrate on.
In practice, things fluctuate like mad. For a start there are all the Google dances and movements in and out of the SERPS - practically every single property has been out of the SERPS at least twice - once when the initial new content honeymoon wore off, and once when some algorithm update or other was taking place. They always came back into the results, occasionally at a slightly higher level than they had been - but the two weeks or so when they were "out" would have been very frustrating had I not had a whole other bunch of stuff going on (the dances seem to take place niche by niche rather than all at once).
Then there's the fact that as the economy and fashion changes, user behaviour changes. With one page I wrote, I sort of panicked when my traffic dropped, only to find that I was in the same place in the SERPs. Checking the Insights for Search graph for the traffic revealed that user popularity for that keyword had dropped. Such is life.
So it's actually worth trying many different niches, and also don't get put off by lowish CPC in the Google Keyword Tool. I'm beginning to understand that traffic matters far more than CPC. CPCs will fluctuate and there is nothing you can about it, but as long as you have traffic, you will get some clicks. Lack of traffic is the killer rather than low CPC.
Finally, there has been much discussion about whether hubpages has suffered a Google penalty of some sort. From looking at my stats, if there is a penalty it exists only in the finance sector - credit cards, debt and the like. My finance hubs are showing a drop in traffic, but the other stuff is much as it was before (i.e. the good hubs show no difference in traffic between July and August and the bad hubs show no difference in traffic between July and August).
I guess, the hub penalty isn't that different to that meted out to Squidoo for weight loss and the like. If you stay clear from the "hot money" topics, you can still get Squidoo lenses ranked, and still get Hubpages ranked.
But these penalties also bring home the importance of having a lot of niches, and lots of platforms where you create content, so your eggs arn't all in a basket that might get kicked over.