Price - Isacord is really cheap. I get mine at bulk from Discount Embroidery Supply, and it is literally *half* the price of Gutermann's, and significantly cheaper than Aurifil.
Ease of ripping - Ripping is inevitable, but I've discovered Isacord is really easy to pick out stitches with. Earlier today I stitched a flower, then decided it didn't look good, so I ripped it out. I was amazed how easy it was to rip. I can probably pull a thread through 10 stitches at once in one swoop, which is much better than if I used a grabbier thread.
Strength - The single best thing about Isacord is how strong it is. (Some might argue that that strength will cut through my quilts ... but I don't worry about that. Quilts are made to be used. I think it's kind of an urban legend anyway until I experience otherwise!) The strength means it just doesn't break. I travel a lot, I stitch over really thick seams, I definitely put a lot of strain on it ... and it doesn't break.
Colors - So many colors to choose from, and a lot of variegated colors to play with!
Hard to secure - The safest way to secure Isacord is to bury the thread. That takes a long time so I don't like to do that, but I've found I can't just stitch over it or take tiny stitches ... it's just too slippery and it will pull out too easily.
Tension - I've rarely run into tension problems with Gutermann's, but with Isacord tension problems are a lot more common. With every new bobbin, I needed to check it out a piece of test sandwich.
Weight & shine - Isacord comes in 40, so it's a bit heavier, and it's also quite shiny, making it stand out on the quilt more. That might be good for some, but I like the quilting to blend in and to lend texture, not to steal the show, and Isacord seems to want to be the star.
Can't piece with - Because Isacord is so slippery, it's not a good choice to piece with, since it's very easy for it to unravel. That means I need to end up switching threads a lot, which is time-consuming.
I'm fairly happy with Isacord despite the cons, but I think I will also try some of the other brands that people seem to highly recommend. The most common one I hear about is Aurifil 50, though it's significantly more expensive, I'll see if it can work for me. If it breaks often, it's a deal breaker.