I have been using one of these for the past three years. On to my second one now. They are extremely useful and I am very much a fan. My first one (a present) was a cheap one. Main problem was that it just went on dispensing hot water until manually stopped. So, you can imagine how many times, I 'forgot' and then had to wipe up that output. So, when it died, I made sure that its replacement DOES have set amounts to dole out. So, have never had this overflow problem now.
So, I would think that is one important facet to look for. Also a good size water tank. My present one is four litres - only me here, yet I still need to fill it at the least twice a day. Mind you, I keep it close to the sink and a small jug which I use just for that purpose - so is not a deadful bind.
Something else that I feel is important is that it has a small tray for the mugs, etc to sit on. These also act as somewhere for drips etc. to be caught and can then easily be emptied, rather than that happen onto your worktop.
My present one also has different heats, (suppodedly to use for different types of drinks). Apart from the few weeks after I first got this - and used these for novelty val;ue -have never used them since. Everything on the one heat - the highest.
I switch mine off when I go to bed, but once turned back on, stays on the whole day, ready for immediate use.
They are an excellent and useful addition to any kitchen and so much safer to use than kettle (I still do have one of these). this requires no lifting for me and only the amount of hot water that I require used at any one time. I also use it for water in saucepans, etc. etc.
Just one word of warning - these are wonderful for one or two mugs of drinks - however when making more at the same time, it is probably best and quicker to use a normal kettle.
I must now admit than when I get my kitchen re-done next year I am planning to move on from even these machines to having a boiling water spout on my kitchen sink.