The ones we have are silent as the grave, and are on 24/7. I asked that same question on Amazon before I bought our first one and an electrical engineering kinda guy (something of that stripe) responded that he had bought one and that they are designed to be left on all the time. So far it's working well for us! Just be sure to get one powerful enough for the load you intend to put on it.
[EDIT - And remember to get a couple of extension cords to use with it - best to get ones that will take three prongs - you can always use them with two-prog plugs, but if your equipment has three prongs, you can't use a two-prong receptacle! It's a lot better to be plugging/unplugging from the extension cord than the unit itself. If that matters. Extension cords are rediculously expensive here, and USA ones are non-existent. Also, for whatever reason, it does not like surge protectors.]