Generally speaking, old countries don't benefit anymore, it's the opposite. The countries that benefit to be in Europe are the ones with a weak economy (or the poor to be more direct) since they get help from EU (not only financially)
Prices increased a lot since, with $5 you can not even buy a piece of fish, of course, the wages didn't follow. Companies started to move to other places.
I somewhat disagree with a bit of it
Countries benefit from it, as they indeed receive more money from ...obviously the richer.
But for the individual, things in the poorest countries are not that bright. Prices have already gone up really bad, and I was amazed to experience the almost same prices in Bulgaria as in Slovakia or Poland, this for basic food while wages are seriously lagging behind.
And the things about companies...they are also moving between those as well with no rule whatsoever as I've seen companies relocating from Romania to Hungary or the Czech Republic or even stranger going back from Poland to the Netherlands.
The euro was probably the best thing that happened, I still have a lot of coins from the '90 and I remember the pain in the ass of changing currency every time, no more calculating each time how much that is in what currency and comparing prices.
No country is singing the same song and the European laws are now more important than the local laws. Otherwise, the country pays a fine to Europe. Most countries are afraid of the EU laws so they obey like a puppy.
This is good in my opinion, as it's damn weird to have some punishment in some country and just over the border that thing to be perfectly legal, I really dislike the US approach on this.