As pointed out, the ISAAK and the Israeli Defense Force A-10/Ephod are fixed-pocket LBE set ups. This can be good or bad, depending on your needs.
My South African P-83 vest is one of my favorite, and is a fixed pocket set up. I designed the MESA vest based on the South African design, but made it completely modular so I could use pouches for my .308 rifle. Each vest serves a purpose.
While modular vests can be less expensive to start out with, you will find that you'll be buying every new pocket that comes out...it's the gear-queer equivalent of the surfer's endless search for the perfect wave.
At retail, I probably have over $350 into my MESA set up. As I find pouches I like better, I sell off the old ones.