I actually prefer this to a hot water bottle or bean bag because the gel pack is light, soft, and mouldable (when pressure applied).
I think this is best used when lying on, so that all the heat is transferred to the body easily when you are pushing down against it, e.g. putting under leg or under lower back. But if you place it on top of your body, for e.g. on your leg, you would have to apply constant pressure with your hand so that the heat radiates continuously. In this case, it would be better to get a heat pack that is designed to strap round your leg instead, which PhysioRoom might do.
I found that my microwave heats the edges of the gel pack more than the centre (this may be due to the way the microwave heats things), so when you take the gel pack out, spread the gel around so that heat spreads all over.
Heating the gel pack at 1000W for 50 seconds, lasts approx 30 mins if you lie down on it, whereas if placed on top of the body (where the heat can escape more easily)it doesn't last near as long. Covering the gel pack will help, which unfortunately Physio do not provide with the pack.
I guess heating the pack for more than 50 secs would give longer lasting heat, but like with any item, there is a limit to how much you can heat it, so you will have to check with PhysioRoom because there is nothing on the pack to say what the max time is. I found that at 50 secs the gel began to bubble, so do be careful.
Lastly, the gel pack is good as a cold pack too. But unlike with the heat pack, it's not mouldable if frozen and does need to be covered up before use. Works great if you simply want it as a cool pack.
Overall, its a useful item to have and i'm glad i bought it.