Can't speak for the immersion heater timer, but for a plug in timer, it depends on what you want. I've got a whole load of mechanical ones which I usually use for my lights as they are easy to adjust the timing as the nights get shorter and longer. I have usually just bought cheap ones and they work fine. I've got one old mechanical timer which doesn't run on time any more but that was up in our garage for many years and probably got water in it.
For things with an accurate set time, I've bought the cheap digital ones. They're not as easy to adjust so are better for something which is fixed, for example I have my battery maintainers come on in my garage at 3am so they don't cause me interference when I'm playing ham radio. I also have my dehumidifier and wine fridge set to come during Economy 7 hours. Again I use a cheap generic timer as shown below, about £6 from Tesco, £8 from Sainsburys, £6.30 from CPC, three for £18 from eBay, occasionally £5 from Middle of Lidl. You need good eyesight (or a magnifying glass!) to set it though.
Oh, I also use these so my FR24 receivers do a 1 minute reset in the middle of the night, and my OGN-R station switches off for a few hours at night.