Are you talking about buying one as part of a CT Putter or installing one yourself (or with help)?

In general graphite shafts can be lighter than steel shafts and in some cases can be much more stiff so it a personal decision like changing shafts in your driver or irons.

IMHO

I had a Stability Plus shaft fitted in a GoLo 5 35" and like the light and stiff feel. There is very little difference in SW from the steel shaft

Have had many graphite shafted putters, just a different feel. I put an LA Golf custom shaft into a png Redwood Anser. I have a SC 009 in the bag, but I’ll leave that as it came from SC, stepless steel shaft.

I’d say try it out before hand! I’m going to be putting the KBS in my squareback! Completely different feel with graphite but I personally love it!

I would love to know how the graphite shaft works in the squareback as I have a stability plus shaft in a golo and was thinking of putting one in a squareback 2 that I have

Changing to a graphite shaft is entirely personal. As has been mentioned, usually a stiffer feel and likely lighter in shaft weight. I swapped in a BGT Stability shaft for my NP2.5+ and any differences were negligible (for me). Fortunately reshafting a putter is something you can DIY if you choose. Which means you can test a graphite shaft and replace with the original steel shaft if the graphite doesn't suit your game.

There are some Scotty Cameron branded graphite shafts out there and the weight etc are the same as steel. The weight is identical to the steel shaft but they look a lot cooler :)

I would and am definitely going to! I currently have the Fujikura Mc in my current gamer and will never go back to a steel shaft now. My buddy has the same exact putter but in a steel shaft and the overall change in feel is huge. Most steel shafted putters feel very "clicky" to me. I just ordered the new 9.2r phantom black and will be throwing another graphite shaft in!

I tried the Fujikura MC Smooth after reading reviews on My Golf Spy. I put it into my Scotty Cameron OG Buttonback NP. I originally thought that I'd use it as a training aid to smooth out the transition of my stroke, especially for longer, lag putts. It worked so well I kept it in and it's been over a year now. My putting is much improved but more importantly I've gained a tremendous amount of confidence and my three putt avoidance has become a strength.

I've also used the BGT Stability shaft in the past but it felt a little too stiff and boardy so I only stuck with it for a few months.

I think trial and error is the best method for putter shafts. My buddies think I'm crazy for trying different putter shafts, but I don't care. It's one of those very personal things that you need to try for yourself before making a judgment.

I put an LA golf shaft on my Newport 2 and its made a world of difference. I wont have another putter without it.

I threw a KBS GPS 120 in my putter and it makes a noticeable difference on lag putts. It's so much stiffer and such low torque that when you whack it 100yds and you miss it toe/heel it will hold the line noticeably better than a cheap softer graphite or stock steel shaft.

I think it definitely makes a difference. Is it enough to justify the price? That's up to you.
I would say I am a pretty above average putter and PGA Pros and my SAM Puttlab stats back that up. It doesn't make a world of difference, maybe it gained me 0.3 strokes putting. I will say I do not regret the investment at all.