I've been using clicker instant head pads for years (I used to buy them from Canada back in the 1990's before they became freely available over here). There are several tricks that I've learnt on how to get the best from them.
The perennial problem so often heard is their practicality of re-use. Way-back I messed around with folding towels and filling the kitchen with steam but on advice from an original Canadian company "Hood', I ALWAYS use a microwave now to re-liquify them and I've not had a failure yet.
The key is to set the timer on the microwave and monitor them. I use an old 650 watt model and either do one at once (bit more hassle), giving it an initial 40 seconds, then turning it over and then give 20 seconds, turning it over after each period of heating until there are no crystals left. It's essential to make sure that all the crystals have gone, otherwise the pad will start to solidify as soon as you turn your back!
I find if you hold the hand warmer up to the light, you can easily see if you've heated it sufficiently, as the final crystals stop swimming around and liquify completely. Once it's clearly liquid, then stop heating and place it somewhere to cool down.
I've got loads of these pads - these Salveos, some from Tescos (blue circles) and other older brands, because I find them so useful to keep one in each pocket of my coats, or in the glove box for my chilly hands having just scraped the ice off the car.
As such, I typically need to recharge several at once. As above, I lay them out on the turntable in the microwave and give them a proportionally greater heating duration. If you put them all with the writing side upwards, then it's a doddle to know which way to turn them "to heat the other side" and avoid potentially damaging hot-spots. It's not rocket-science, but some pads, even with the turntable active, will get to the liquid state quicker than others. As before, I monitor them and hoick them out as they're done.
If you don't need the heat instantly, by keeping the liquid (cold-state) pads in your pocket or elsewhere about your person before clicking them, so that they're "pre-warmed", you'll be amazed at how much hotter the pad becomes when you do click the clicker. Be careful if you do this, because there's a chance they'll be too hot to touch and you'll need to wrap the pad in something once the solution activates. I've found that irrespective of wrapping the pad (which in itself will prolong the heating effect), pre-heating the pad definitely means that it retains its heat for longer. Let me clarify - by pre-heating, all I really mean is taking the chill off - the pad will still feel cool, just not as cold as if you hadn't done anything with it before clicking.
If you've more than a couple of pads, consider warming a fresh pad against a "dying" one - again, I find this greatly extends the potential heating duration.
All-in-all, for instant heat, pads such as these are brilliant in my opinion, if you realise their limitations and how to get the best out of them.
For longer-term hand warmth, I use the Zippo hand-warmers (or sequential mugs of cocoa - but this isn't always practical when you're out and about)!