180g 300s M5. I am a huge fan of prodigy mids and have an M1, M3, and now an M5. Obviously the M5 is the most understable in the bunch. Thrown "hard" it will turn right and never come back, but it's not gonna turn and burn on you unless you full power rip it with improper form and some anheiser (I say "hard" because that depends on the person, form, and arm strength. I don't have the strongest back hand in the world.. like 350' max). You can really shape almost any shot you want with the M5. I love it for longer hyzer shots that need finesse. I can throw it on a moderately steep hyzer angle and it flip up to flat and soar a waise, then fade left. Thrown flat it makes a good turnover mid. The "double rim" that everyone complains about does not affect my grip at all and it is hardly noticeable. All in all great disc. If you throw mids alot, this could fill a key hole in your bag, I know it did for me!

Update: After a few months playing with this disc some things have changed. First of all, for this disc i would highly recommend 400 plastic. The 300 plastic broke in pretty fast and is now very understable. Thrown with some pretty steep hyzer i can still flip it all the way over to the ground on a strong throw. Takes alot of touch to play with, but still use it alot on downhil shots I need to go right. Great for short range left to right shots as well. Cannot put any anhyzer on it unless thrown very short. Will turn and burn. One of these days I may try a 400 M5 but for now I will stick with this one. This is one of the few (if not only) truly understable mids.

Update 2: I purchased a 180g 400G M5 about 2 weeks ago and after throwing it several times I really like the consistency I can get. Flies about like the 300s did right out of the box, thrown with some grunt it will never come back, but I can get about 250 - 300' out of it. Thrown with a good amount of hyzer it will flatten out and then fade at the end. Highly recommend getting the 400G over 300S in this disc although I still bag the 300s... flies like you would expect an M7 to fly.

The M5 is one of the most versatile discs in my bag.  Mine is a 170g in 300S plastic.  The plastic is very firm and the top of the disc is board flat, like a McPro Roc3, but is not as slick and is more grippable than the McPro plastic.  Really excellent baseline plastic!

The "groove" is not like the atrocity of the Innova Groove or Monarch;  it's like someone carved an indention into the plastic when it was soft.  It is not a problem at all.  The rim *is* a little thicker than most, but I have no problem with that, either.

My home course has two dogleg-rights, one uphill and one downhill, both in the woods.  I wanted a disc I could throw RHBH and have it turn over.  On the downhill, I throw it straight and flat and it "breaks" right and follows the fairway to a smooth finish, usually for tap-in birdies.  If I throw a little high on the anny shot, it'll S-curve at the end and finish softly.  On the uphill, I throw it straight but with the nose turned up.  It flies straight then curves right for a nice birdie look.

I also threw this disc in some strong right-to-left crosswinds that just eat up more stable discs (such as the Roc3).  The M5 fought the wind extremely well, flying very straight.  Directly into the wind, it will turn over.

The M5 is also a great approach disc.  It glides fairly straight at less than full power, or a helix shot can be shaped with it.  It is extremely versatile, and worth the time to work with it and learn its nuances.  This disc is in my Core, permanent bag and I heartily recommend giving it a try.

This disc is an excellent disc for Anhyzer shots. I have found the double rim to be a bit bulky making it a little uncomfortable in my hands. I'm probably just not use to the disc yet though.  Definitely would recommend picking one up and giving it a try. 

Update: I have thrown this disc more and have found the disc to more understate than I originally thought. I can get excellent Anhyzer flex shots with this. I still don't find it to be very comfortable in my hand though.

Prodigy did it. They got a midrange to actually be understable. I tested a 174g M5 in the 300S plastic. This is a great disc for moderate throws that you want to turn slightly but overall go straight. This disc will fade at the end of the flight, but not much. It's understable, but not too flippy that it immediately crashes into the ground. The ultra grippy 300S plastic is perfect for anhyzer throws on short shots. This is a solid understable mid.