I use these as my main putters because it helps fight the error I make by releasing on too mych hyzer.  I think someone used to a very overstable putter would have to adjust.  I find it holds the line well and is great for anny putting, as noted by everyone.  Also a good understable thrower beginners might love for straight shots and learning angle control to work the disc.  Could one disc a pitch and putt course.  The numbers seem about right with the usual caveats about wear and premium plastics adding overstability.

My only knock against the Zero Medium Keystone is the plastic getting beat up so much relative to other plastic types. You'd think a putter wouldn't get so worn. But even with the wear, the flight gives me a good alternative to my more stable putters. I've used it as a throwing putter at times for its reliable flight and higher glide: it goes mostly straight with some pull to the right (RHBH) and remains pretty level even during its turn and fade phases, so it parks itself nicely instead of skipping or rolling. I like using it for putting in tailwinds as I get a straighter result than my other putters. I got the Retro Burst version for my brother for his first set of discs last Christmas, and I've seen it work really well for him as a beginner, both for putting and throwing.

Understable putter holds the anny its put on. I like it but i like the proxy better, mainly because the proxy feels better off the box. and i like the hand feel of the proxy better. If u like i deeper rimmed understable putter, it will do. 

Until Latitude makes a disc that can correct user error mid-flight, this is the next best thing. The Keystone is a very forgiving disc. Despite the high glide factor, it hunts targets with greater precision than Jason Bourne. The mold is very ergonomic -- especially intuitive for those crossing over from Ultimate. When new, these will hold a straight flight up to 150. But decently seasoned in New England forests, it will draw right with the grace of a Corey Pavin 3-wood circa 1997. The Keystone is not only my go-to disc for fairly straight upshots, it is also the only putter I use w/in Circle 1. The plastic feel is nearly perfect -- just the right amount of tack. And the glide works well for my modified push putt. Go ahead, turn your next basket into an unwitting Noah Vosen!

Zero medium 173g. The blunt round rim is super comfortable. It seems like a deep dish but when compared to other putters it is a medium dish. I could see this mold getting more popular over time

Throwing: I would describe the flight as... flip to flat... straight... straight... floaty glidey straightness. It has very little fade. On some throws with a little wind I saw a smidgen of turn on a hyzer flip but nothing crazy. 

For putting: It is point and shoot and comfortable in the hand. Straight. Highly recommend.

175 Zero Medium

These are my absolute favorite putters. They feel so good in the hand and just go straight when putting. If you want to throw them they are also fairly straight with a little bit of right finish if you give them enough power. I wouldn't recommend driving with Zero Medium ones as they can get pretty flippy, but they are pretty good for driving if you get on in Opto

Turnover, turnover, turnover. When a little beat in this disc holds turnover lines like a beauty, and fits great in the hand. A little faster than 2, maybe not quite a 3. Great for throw-ins as well!

I enjoyed using the Keystone in the Trilogy Challenge, though not enought to make it become my new putter. What did stand out to me is how much glide it has! The glide 5 reminded me of the Macana. It's a very straight putting putter, and feels nice in the hand.

When driving, it flips over unless you put some touch on it. It'll hold the line you throw it on, which is very nice! It's a bit rounded, but not too deep. This is a great over all putter!

After the Trilogy Challenge, this is going to be my putting putter. The Keystone is only slightly understable; it'll hold a right turn before straightening out at the end. It won't turn and burn unless you put a lot of power behind it. I really like the glide; it makes putting from distance a bit easier and since it's lightly understable, if I put it on a light anhyzer line, it will hold it longer than my Wardens.

I prefer the rim, but that comes down to personal preference. It's very rounded, not terribly deep, but not shallow like a Pure. Overall, I really like this putter especially since it's going to be my new putting putter. The Retro Plastic also feels really good and grippy.

I'm split on this disc, I liked it to 3 stars, but the performance for myself and others make me think it's really a 4. The rim is very symmetrical with a flattish top slight angled slope down to a blunt rim and then it angles almost identically on the bottom edge. 

On putts I liked how it was very straight on a spin putt and you didn't have to put a ton of pop on it and it went where i pointed. The wind effected it a good bit especially on closer to full power throws. If given some hyzer it flips up and rides really well and I had several throws over 300ft with it and it would stay straight and not fade much. If you throw it flat with good power it will likely turn gently and never return and attempt to fade very much or at all. 

Overall it's a good disc but the rim shape isn't my favorite and led myself and others to griplock it occasionally.