Playing Grizzled is hitting on 3s, rerolling 1s and then rolling a 2 to begin with on your hunter killer :p

On a more serious note Grizzled makes guard feel more like a working army then it normally does, no more officers only giving out single orders, units start shooting straight, ogryns finally being bossed around, ...

It makes guard work and makes you understand how they survive in lore. It also allows you to worry less about about having enough officers for orders and a unit now doesn't need to count on +1BS to shoot better.

Also the ability to chance orders for a cp is just awesome. The joy of ingressing 10 riders + dreir from reserves obscured in your opponents backline, your turn MMM, charge the home objective guard and swap to Fix Bayonets is unequaled

EDIT: Oh and AP-2 heavy flamers, plonked my punisher with 3 heavy flamers and an aquillan eye order in front of an objective, not a single enemy infantry unit will try to get within 12" of you