The benefits are mainly 1. insulation/air sealing - they make a very tight, efficient structure 2. Strength - SIPs typically make for a stronger structure relative to conventional stick framing (I've seen picture of big trees sitting on SIPs houses with very little damage, for example) and 3. Speed of assembly.
They go up very fast - much faster than framing and sheathing, but that will only hold in an apples to apples comparison. 2 guys doing a house with SIPS aren't going to beat a crew of a dozen or more guys framing. With a reasonable size crew they go up bam bam bam. And they tend to come out quite straight, if the crew is competent at all.
Downsides are cost is a bit higher and some things are more difficult - wiring and plumbing (if you put plumbing in exterior walls - not done much up north here), and any last minute changes can be somewhere between annoying and painful, and some changes just cannot realistically be done to a given plan once you have panels on site. They do require better planning ahead as you need to order panels that will be made to fit.