My guild in World of Warcraft is level 24, and will reach level 25 soon. And the most remarkable thing about that for me is how little I actually noticed it. Some of the advantages are downright invisible, because nobody really notices an X% bonus to this or that. Others are weird: Why did Blizzard make the heirlooms you pay for with justice points rather expensive, and at the same time offers new heirlooms as guild perk which are extremely cheap in comparison? The one perk that ended up being most useful to some of my characters was the 15-minute hearthstone.
Apart from a few trade chat guild advertisements stating the level of the guild I haven't really noticed much of a change in guild behavior due to the introduction of guild perks. I have no idea whether they are working as intended to reduce guild-hopping. Maybe my guild is just too nice and stable to react to perks a lot after 6 years of staying together without rewards.
So I'd be interested in hearing from you whether you noticed any effect from guild perks. When they were announced people theorized all sorts of effects from being stuck with a bad guild to a positive effect on guild loyalty. Did you experience any of that? Have guild perks as a means of social engineering worked? Did they lead to small guilds dying out, replaced by anonymous mega-guilds? Or did guild perks in fact not change anything fundamental?