Great for heavy periods, heat-sealed seams are EXTREMELY weak.
I have heavy flow during the first 2-3 days of my period. I got tired of washing (destaining)  blood-stained clothes so I got these vinyl pull on under pants. Since I usually wear a size 8/Med in clothes, I got a size Med. Boy, they're huge! The elastics are not that uncomfortable but the vinyl is super roomy. It balloons out as if your tummy and hip are supposed to be fluffy airy bread/marshmallow. It doesn't matter though. They're under my clothes (pants/shorts/skirt/dress). As long as the blood stays inside the vinyl, what does it matter what it looks like?<br /><br />I do a 3-layer protection during heavy days - overnight pads (or 2 overlapping heavy pads for a really long overnight pad) worn in snug panties + vinyl under pants + a waterproof incontinence mattress pad. The vinyl under pants catch leakage/spillover. The mattress pad is just there in case the vinyl under pants fail (but that hasn't happened yet). These are vinyl so you're going to perspire a bit under it. But, I only wear these for 2-3 days per months so it's not that big of a deal. I don't know how comfortable/uncomfortable this would be if I have to wear these 24/7.<br /><br />There's less to wash for me now during my heavy days - just underwear, no pants/shorts/skirt/dress. I only hand wash these vinyl under pants and then hang them to dry.<br /><br />Update 10/25/2015:<br /><br />What others have said is true. The seams are really very weak. After a few uses, the seams come undone. Downgraded to 2 stars. What I did to fix it was to use a soldering iron very lightly and over a PTFE cloth (oven liners) so the soldering iron will not stick to the vinyl. Be very careful when you do this. You only need a light tough otherwise your iron will melt the vinyl through.<br /><br />If you don't want the hassle of using a soldering iron, just apply heavy duty shipping/packaging clear tape. It doesn't look as neat as hot sealing but it's underneath your clothes. Who cares what it looks like?<br /><br />Another option is to use vinyl adhesive - e.g., PVC cement (the kind used in plumbing) or Loctite vinyl/fabric/plastic flexible adhesive. Apply with a cotton bud. A good preemptive step is to reinforce the seams by adhering the seam with vinyl adhesive so that it doesn't come apart when the hot seal eventually fails (and it will fail). The PVC cement is very strong so use just a small amount and spread evenly over the seam surface.
Rating: 1 stars