If I recall, the aluminum is about a 40 lb weight loss and will free up a few (2-4?) RWHP. As stated, it'll help your 1/4 time but MAY be negligible as far as SOP feel. I've written it off from my list (so far) due to the possibility if install issues and the lack of bang-for-the-buck. Others will disagree.

 

I got my Shaftmasters for $450 shipped by just calling them. It is a 4", however I have zero plans of ever lowering the car (not my bag). They may be able to make a 3.5", just call and ask.

When I see prices of $800+ for these things I have to laugh. Mine has performed flawlessly and has zero vibration or any other problems whatsoever. I was on the fence about getting one, but the change in "feel" of the driveline in terms of removing ALL slack and making the car feel completely connected was totally worth the money. I'd do that mod again in a heartbeat. Its hard to explain, but the car now feels like it has no boat anchor attached to the back of it. I never thought it did before I got the driveshaft, but that was the case. I don't know what difference it makes in terms of supposed weight trade-off vs horsepower and track times, but none of that matters to me. The car feels way better now.

 

I had a coast 4" DS on my car but I didn't like it so I went back to the stock one.
I mostly didn't like the sound aspect.
Basically, you have a big aluminum tube down there resonating all the noises of the car.
The exhaust sounded completely different. A lot more drone and more of a hollow sound to it. I have Flowmaster axle back which is not normally know for much drone at all.
Also there were vibration issues at around 80+
Pinion angle adjustments solved most of this but there was still some annoying vibrations...
There was a slight bit of rubbing on part of the under-body but that was under extreme suspension travel. 100mph hitting a dip in the road...

Overall, I just didn't enjoy driving the car as much so I took it out.
Still sitting in my garage actually...

 

Hmm, I didn't notice any new sounds with mine at all. I wonder if the Coast vs Shaftmasters units are constructed differently somehow...

 

I have the Steeda driveshaft. Car seemed to rev a little quicker and feels a little faster off the line. I'll know more in a couple months when I go back to the track. The only time I notice any kind of vibration is when I get up to around 125, even then it's slight

 

+1 depends on how far you are lowered and as stated the cable adjustment is a suggestion, not required.