gives 80 ap, 20% cdr, 600 mana and the active all for 2800 gold. ludens givesm90 ap, same cdr, same mana and the passive all for 3200 gold. pros are starting to buy it because its much more gold efficient than ludens and much cheaper
its definitely not sleeper it's one of the best items on a couple of mages ahri, veigar, swain, neeko are one of the main abusers of the item i've even seen brand use it to guarantee his combo pinkward was going either glp or protobelt on ap shaco as well
paired with GA it's busted if they ever decide to buff the active in some form it will be the best item in the game
welcome to 12 months ago..........
Its already bought a lot together with Glacial rune. Ludens has overall more dps/waveclear still, so unless you can use the slow well and often, its not worth it
It's a magnificent item. But I wouldn't call it "sleeper"
Unless someone only judges by their soloq low elo games.
The fact that it's only 2800g is the main reason it's so valued in high elo. It let's you hit your cdr and 1 item powerspike with sorc boots sooner. And it let's you move to the next item sooner.
It's build path is also EXTREMELY convenient and paired with TP you just have complete control over when you back (Just buy a million amp tomes 4head!). The 
on the luden recipe means you NEED that 850g back timing and then you NEED to back with 1050 to complete.
Meanwhile GLP can be broken down to pretty much 500g steps.
Base components are just 
finish costs are all super low. 180 (I think) for revolver (with a nice trading passive). 80 for chapter. and just 450g for the entire item recipe.
I really like GLP on Sylas, but unfortunately I can't go protobelt and GLP, kinda stupid.
