I agree. The E-Series is one of the most adaptable out there. And - the P series isn't a slouch either.
On the E-Series, you can choose different bodies, tailcaps, heads, and adapters. Some people even sell weapons mount for the E-Series as well, and Aleph and Vital Gear both sell aftermarket bodies for use as well.
On the P series (6P/9P/G2/G3/etc), you can get different heads, drop ins, converters/adapters, and tailcaps too.
It's all adaptable. It's fun, and it's addictive.
-Steve