The horror stories I've heard about the new nids are just stupid and even make my Nids buddy not like them for being cheesy and the fact that they have officially made it "Tank or die" (or at least put the final nail in the coffen) kind of saddens me from a fluff point of view because the Knights have fought worse and don't need tanks.
The new 'nids are not cheesy in the least. With the exception of gribblies, costs went up across the board for the entire army. And if GW didn't give the 'nids can openers -- some way to deal with tanks -- the army wouldn't sell, period. They will still struggle against modern mech armies, but at least they won't lose automatically anymore like the old 4th edition 'nids codex does.
The reason people should be mechanizing is because the base rules make that the better choice! Seriously, how can you not look at the vehicle damage chart -- especially compared to 4th edition -- and not draw this obvious conclusion? Combine this with how cheap all vehicles are getting (in the new codexes), and isn't the mechanization of 40K an obvious extension of the basic rules? The game has been
designed to encourage mechanization. GW wants to sell you models, my friend!
Beyond that, what is "unfluffy" about mechanization? Think about modern warfare. How survivable are infantry on the ground? Right, then. This is why there has been all this hullaballoo in the US (and other countries caught up in Iraq and Afghanistan, just to mention the most recent history) about improving the armoured transports for troops. (And look back to WWII. Did you laugh at Poland for trying to defeat Panzers with cavalry?) You really think SF warfare in the 41st millenium is "fluffier" by being about a bunch of guys on the ground with very little involvement from vehicles? Don't be silly.
If you want to continue to play with a foot army, that's your choice. But don't do so thinking "codex creep" and "cheese". Foot army players are living in the past and failing to adapt. The game has evolved. Evolve with it ... or lose.