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I've got a few of differing low budget makes and types, from £15-£50. They sometimes flag up an issue that you might have overlooked but they are not as important as experience. Most of the mistakes people make are avoidable. If you're not sure go slow and use smaller bits, investigate a bit. The finders help if you are outside of your usual property/construction type and trying to figure out what you are drilling into.
For example you already know all the zones for cables so you should always check in them if you had to hang a wall cupboard. That's a scenario where a cable finder can give you a big warning that there's power in that area.
Unless the property looks like the electrics haven't been updated since the 60's. In those cases you need to think about checking if you have any rcd protection as well.
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Only one I've ever used that was any good was this one from Bosch but it's not cheap.
I'm also interested in people's experience with stud/pipe finders. Doesn't have to be budget, but I'm not about to drop a few hundred quid on a top end one.