I love square. It got me back into photography big time after an on and off hiatus. I don't crop 4x5, however... I use a Mamiya C330. Mostly Provia 400X film, I'm really happy with the combination. Sometimes I stitch two 6x6 frames together.
The square format lends itself to one type of composition: center-the-subject. Nothing wrong with that strategy, but it does make static pictures. While I've seen plenty of 6x6 shooters push and stretch that frame, it always wants to come back to the center.
Maybe that's the allure of the square: battle between the gremlins of centering and the fight to make dynamic pictures.
Hmm... that's a bit harsh, I think. Other aspect ratios can be just as bland.
Here's some very random images I was happy with in 6x6... I actually find it quite challenging to find compositions that work in that format. For me, it works better with still life and portraiture. I see landscapes wider.
One of my 6x9 cassettes for my Arca has an insert to crop to 6x6. I've only ever used it for one catalog production, when we were shooting lots and lots of small objects (what they use digital for these days). Somehow the incentive to use the insert and 6x6 is not big enough, especially since taking 12 pictures in large format might take a long time for me now.
I do like 6x6 with a TLR a lot though.
Square is fantastic; until I got back into LF three years ago, I only shot squares.
You can see them if you visit my website.
I shot people, people and more people, and it's a lovely format for portraiture, whether or not you want to compose formally.
The square can be lovely, and using a terrific camera like a Rolleiflex can be much of the reason the experience is a good one. Some stuff fits nicely into that frame.
I have a couple of books that are collections of square photographs, and for some reason, viewing the pictures can get monotonous, at least to me.
Helmut Newton used to talk about using medium format in his fashion and portrait work. He usually planned to crop off the sides to give him a vertical rectangle.
Years later, reviewing his old work, he found he was fascinated by some of the stuff going on at those edges. He liked it and started to include it.
At the risk of letting this thread veer farther out of bounds into medium format, I can't resist posting a link to the work of Victor Ben Zvi. He uses a Rolleiflex and employs the square for his portraits and landscapes:
A TLR is a great way to shoot people. I love the square format for this purpose. I also like how subject interaction works with a TLR, the way the camera is operated and held low, and the sheer beauty of the things (which of course means nothing for the photo).
For the things I shoot with large format, I don't think I'd like the square format. It should be easy to mask the ground glass though, and if you are a purist (or have other reasons) you could make a square dark slide to get squares on the negatives/slides.