I thought I'd throw in my 2 cents, as someone who's only really started watching Kamen Rider.
I started Kamen Rider W, when it was up to Episode 20, and fell in love with it straight away. It has good writing, fantastic action, and is a really good series.
I was going to start Decade, but after talking to a few guys about it, I felt I'd miss out on a bit of the fun of Decade, as I wouldn't know who anyone was, and so Den-O was suggested, as I was told the humour was similar to W.
I'm about 14 episodes in to Den-O at the moment...and I'm having trouble with it. It's more humour based than a lot of the series are supposed to be, but...it seems lacking, especially compared to Den-O.
Thanks to W, I plan on going back and watching all the series, and then attempting Decade, but I'm not sure I would have felt the same if I started with Den-O.
(and that's how I started watching Kamen Rider)