I actually just received one for Xmas (swag from Roche Pharma) and bought one for £6 at the local "hardware/DIY shop." In the end, they both keep coffee warm for about 12 hours and don't contain glass like some of the other products on the market.

However, one has a lip so that when I screw back on the cup/cap the remnants cannot drip over the side, while the other doesn't have this, so the sides are always filthy.

You really need to try before you buy or have a B&M store with a return policy.