The competition is fierce. The chances of ranking in the SERPs in a decent position is near impossible. And leads are costing a FORTUNE and most aren't converting as high as other niches because they're bombarded with promos all the time.
Put it this way, the marketing niche on Facebook is costing around $1 per lead. Not too bad I know, especially when some are then nurtured into buyers.
HOWEVER, an alternative niche, that's not as heavily promoted to or as commercial and that's actually more profitable is costing around 25¢ per lead.
This happens in all forms of lead gen: Banners ads, Adwords, SEO and so on...
Far too many newbies are taking the 'easy' option and starting out marketing in the marketing niche.
The truth is that most other niches (besides the obvious such as weight loss) would be easy to break into, faster to monetise and more profitable.
So before you start yet another marketing blog on the internet giving away another marketing guidebook to build a list, think about what you're doing and pick something that others might not have thought about, a group of people who might not have seen 1001 squeeze page.
Niche selection isn't as big of a deal as people make out. Ultimately everything in life and in the world is a niche or part of a niche.
[Rant, of sorts, over]