Swinging flat makes no sense unless you flatten your lie angles…
I have yet to find any disadvantage to swinging the golf club as flat as possible.
It just makes the entire process much easier to swing flat. It makes golf a lot easier.
Uses to swing flat stuff years ago…got away from that…think I’ll do some bending myself for the upcoming season. Won’t be using the flat stuff everyday cuz the more active pivot engagement will cause me to lose too much body weight during the summer- too much work in the sun 
Has anyone considered the effect of flattening equipment with how it will affect your wrist set @ address from what you’re used to? i.e perhaps from slighty down to slightly up; and how is the difference remedied if at all?
We consider all those things in the ABS class here…
But there are several ways to deal with flatter lie angles…
If you do it … you’ll find it quite natural to swing on a shallower plane…
You can bend your knees more…turn flatter, and also enjoy hitting the ball straighter.
Lag,
Is there a certain degree of lie angle you recommend prospective students to start out with?
3JACK
Good question Richie. I started this journey at 2* flat and I’m moving to 5* asap… but that’s just me.
Lets Bump this bad boy and see what others think…
I just heard back from my club builder… I can’t bend my beloved G40’s past 3* flat or else risk cracking and breakage… 
Looks like I’m venturing into the forged arena for the first time! I’m a club ho’ at heart anyway!! 
[b]PS: The builder I used for the G40’s asked why on earth I would want irons so flat? To quote him: “PING DOESN’T EVEN GO THAT FLAT… JUST ADJUST YOUR SETUP TO FIT THE CLUBS!”
I replied with “Take care, brush your hair…”
I’m off to Ebay!!![/b]
Awhile ago I wanted to get my Mizuno’s bent 2* upright (yeah, I know…mistake) and the PGA Superstore was nervous about doing that and tried to talk me out of it. Clubfitters are generally that way. Although with the Pings, they would crack since they are cast. But with a forged club, especially a Mizuno which is a super soft forged steel, you should be able to bend them just about until your heart is content.
3JACK
I only managed to flatten my irons 2* flatter in Melbourne. The guy just didn’t want to do it more than that. Maybe I should insist harder. I’ll try again next week.
My flattest is 4*, DX1 imported from the guru himself. Really nice!! I’m still a hacker though but I still remember the day I only missed 1 green from 4 par 3 using DX1. Not really missed it though because I intentionally tried to put the ball right beside the green because water is everywhere. Then again, I want to experiment with flatter than 4*… thinking of buying bending machine but still out of reached.
I have had a 1950 spalding 3 iron bent 12 degrees. And just this week got my last set of Cast irons not sold yet (this is actually my first set of golf clubs and I dont want to part with them) was bent 4 flat. I had to give it in writing to the guy that its OK if they break. These are a no non sense cavity backs something like the Nike Reds. The photo is stock. Mine have TT LT steel shafts.
I think there’s a large segment of the ABS population who, due to weather and/or time constraints aren’t playing a lot. I think Macs approach to it is an excellent option for them and for those who aren’t sure how flat to go and who have clubmakers that are so nervous about bending an iron. Get an old 2 iron and 6 iron off ebay (or some used club barrell somewhere) and use them exclusively for your practice sessions. You’ll get plenty of feedback, you’re not incurring the cost of bending 10 irons, and you’ll probably get less push back from the repair guy about breaking one old iron (which won’t happen anyway). When you settle in on the right number then commit an entire set.
Go to your local thrift store like Salvation Army or Value Village (In Canada). I have bought quite a few 2/3 irons for like $ 1.99 a piece. You can bend them in a steel door jam or something. I keep one in the garage, one near the bag and I think one is burried in the snow in the backyard.