Couldn't manage without them but, we are graphic artists.
If you just want a mouse substitute, don't bother - they aren't equivalent even though you can match the actions of one with another.
If you do any drawing or graphics of any description these pads are definitely the best thing to use.
Please don't buy a cheap one - you will not get the best out of it. I suggest one of the Wacom tablets - not the cheapest but they are the best. For general use an A6 size is quite suitable but, a larger size is better if you can afford it.