Timely thread, just this week I was looking into ear plugs.
I have the general fit "musician ear plugs" that sell for $14-$20, plus a generic version of that I got for $2. Different companies make basically the same product, made of silicon with the 'pine tree shaped flanges'. They are fine, for rock type volumes. I also use them to attend other people's shows and at parties. Two are not comfortable enough for me to wear for more than 30-45 minutes without taking them out and re-inserting them. The other pair I can wear for over an hour, but I still feel them. These generic fit ones can itch the sensitive skin inside the ear. I don't use them for my jazz quartet gigs as we keep our volume low.
I'm going to visit an audiologist next week to get moulds made for custom plugs. Why not? It's not expensive in the long run and is protecting the great asset of my hearing. I've read and heard that they are far more comfortable to wear, in addition to offering better acoustic fidelity.
One thing I notice is that practicing double bass arco in a regular size bed/office room is quite loud, and after 45 minutes can be tiring on the ears. I think the 9db filters on custom fit ear plugs is a great thing to have for DB (arco), cello, violin, viola, acoustic piano, and other such players who need to make acoustic sound wth deliberate attention to timbre, over many hours in a small room. Acoustic instruments can be loud.
5 years from now, I'm sure I won't miss the money saved in not buying custom ear plugs, but I will appreciate and use the custom fit plugs for those times I need it. The audiologists you visit will likely have many options to demonstrate for you. Most custom musician plugs have the mould for each ear, and then replaceable filters, with the choices being -9db, -15db, and -25db. You might get most use from the -9db filter if you most often do fine with no ear plugs. I will likely choose the -15db filter. There are some plugs where the filters are not removable, but are perhaps half the price of the other kinds. If you plan on choosing one filter and sticking with it, those types of plugs could be a better option.
I'm not so convinced about the -25db filters for custom moulds, for non-drummers. If the group has to lower volume by that much, chances are the sound is poor anyways and the 'acoustic flavouring' produced by generic fit silicon plugs will be of little importance, so I'll just use generics when needed. But the physical comfort issue is still relevant, and so those -25db filters might still be desirable for repeat loud-band players. ;-)
I also keep some generic foam plugs in every gig bag, and every coat inside pocket, just in case.