I agree with most of what has been said, and, after not beeing happy enough with my Singer CG 590, bought myself a sailrite LZS-1 about 1 1/2 years ago. Needless to say I'm very happy with this machine!
There is 4 things I would like to add to what was said before:
1. Walking-Foot: certainly, it helps to handle more layers. But, and this I find even more important: with large stiff pieces of canvas, like Sails for example, you've got so much more control over what you are doing and how your results turn out. Many things you still can do without a walking-foot, but it's getting very hard on you and stressful. With the proper walking-foot you are able to get a straight line where you want it and stay relaxed while you're on the job.
2. Doublesided Tape: for everything heavy and big I nowadays always use doublesided tape. I can set up the material how I want it, and it will stay there till the seam is done.
3. Zigzag: triple-step zigzag might look nice, but 2 rows of normal zigzag are a lot stronger. For everything heavy duty you really only need straight-stitch and zigzag.
4. Size of the arm: certainly, I would love to have a longer arm, but there are limits in what I can carry and limits to where I can stow the machine. So this one has to do. And, surprisingly, it does!
There does not exist a sailmaker around here, but often people come in and have a tear or whatever they would love to have repaired. They come and ask, and of course I help them out. I insist we do it together, so they have to help. After spreading out the sail and sticking on the patches from both sides (with doublesided tape) we roll the sail up very tight on the side where we have to feed it through the machine, carry it over, and feed it through together. The largest sails so far I have repaired was an 80 m2 Genoa where the seams were gone in several places across, new sail-battens on a 55 m2 mainsail and a tear in a large squaresail of a squarerigger with 6 layers of 12 oz cloth. As I said, we were with 2 people and the cloth had to be rolled up really tight, but we managed and everybody was happy.
The Singer I'm gonna keep for tiny things like getting a patch on one of my jeans or replacing a zip in a pair of trousers as the machine is less clumsy for things like that. But for the real stuff my beloved Sailrite is the one to go!
Cheers & happy sewing Dody