Is an expensive paintbrush worth its price?

This is a complex question with answers that do not match its complexity. In my experience as a professional residential home painter I've found that in most situations a less expensive paint brush, or really most any painting tool, would work just as well as a "top of the line" version of that tool if only being used a couple of times, and without rigid time constraints. What sets quality apart from generic is in a quality paint brush's re-usability over a long period of time, and speed at which a re-used brush can produce a crisp, fine, and even paint job. Meaning if you are planning to DIY your next living room paint job a relatively cheap set up brushes would probably do. If you were then planning to paint the remainder of the house, and your neighbor's house as well, and needed to do so in less than a month then a top quality paint brush would be the better choice. With proper care, diligent cleaning, and correct painting techniques a high quality paint brush can remain an effective tool in the painter's arsenal for 25 years or more.

It depends on what's your definition of expensive.

Very expensive brushes are not really necessary if you know how to choose medium priced brushes.

I have like more than 20 brushes, synthetic and naturals, of good quality but reasonable priced, and they do as good as the more expensive brushes.

Also more expensive not necessarily means better since some brands charge you just for the name. So consults with other artists before spending your money.

Of course it doesn't hurt to have a couple of high quality brushes if you can afford them, at least for fine lines and small details. But remember that expensive natural hair brushes (like red sable) need special care or they'll be eaten away by very tiny bugs, like those that eat clothing.

It depends on what type of brush and for what type of painting. A sable brush is very expensive but the best for watercolor. A pig bristle brush is very expensive but very good for oil -painting. These brushes can last many years if you take good care of them. Work with poor materials can be a real detriment to a serious painter.

An expensive paintbrush might be more durable.

You get what you pay for... unless you're relying on brand name only, where you get "took" most often. ANY mid-level quality brush will last a very long time if you LEARN to take proper care of it.

Yes. The bristles are less likely to fray and/or fall out. Make sure to thoroughly clean and dry brushes after use.

I think this depends on what you want it for. I've got some ragged old brushes because ragged old brushes are very good for certain jobs!

If it is made of equally great quality and you will be using it for its life rather than a one time thing

Yes, take care of it.

Only if you take care of it.