There are many different levels of "premium" classes.
- The most basic is that you get a glass of sparkling wine and a meal while the people in coach don't, but the seating is exactly the same. Some airlines used to just have a (movable) curtain separating the two classes, and this was mocked in this ad
- The next higher level is that they block the middle seat in each row of 3, so you have some extra elbow room. Example
- Then, the same thing, but the elbow rests are moved a bit so you have a bit more space at waist level.
- Then, you get actually larger seats (e.g. 2 seats where they would usually have 3). This is measured in width and "pitch" (the distance between two rows). Example
- Then, they increase how much the seats can recline
- Then, you get to the point the seat reclines to fully flat, but not horizontal Example, see business class
- Then fully flat and horizontal
- Then size/space increases
- Until you get to a fully enclosed "suite" with a bed.
There are also differences in equipment, mostly the IFE (in-flight entertainment) which can vary from shared overhead screens to 20"+ personal screens with hundreds of movies and shows at your disposal, as well as catering.
So, most definitely, your experience may vary a lot. It depends on the airline, class (premium economy, business, first, suites...), short-haul v. long-haul (note that for some airlines, "regional" can cover flights of 6+ hours but still offer the low-end product), the aircraft type, the generation of the aircraft (they don't refurbish/upgrade all aircraft overnight when they introduce a better product), and more.
"Premium economy" on some flights may be better than "First" on others (by far).
Some airlines use different names for different products, or qualify them (with "regional" or "domestic/international" or "short haul/long haul" or "Euro/World"), some will just say "First" or "Business" for any flight even if First on one flight is completely different from First on another.
You can have an idea of the type of comfort you'll get by looking at sites such as seatguru.com which will show you a map of the cabin and give you details (such as width, pitch, type of seat, reclining angle, equipment, etc.). You'll notice that many airlines have different products, and even the same type of aircraft (e.g. 777) can have many different variants. You can look up what type of aircraft will fly which route, but it can be tricky to know which variant exactly in some cases. And of course there's the issue of the last minute aircraft switch.
The best experience IMHO is long-haul first class on South-East-Asian carriers such as Singapore Airlines or Cathay Pacific. Some Middle-East carriers are also supposed to have very good products but I've never experienced them.
On the better airlines, even long-haul business class is just a significantly better experience. But still not cheap, we're usually talking a few thousand $/€/£.
Of course on some flights/airlines you can get the lowest end of premium classes for not much more than economy, but it will rarely be an "experience". It'll be a bit better, but often not worth the extra cost (in many cases people take that for the flexibility of the fares rather than the better product).
If you really want to enjoy a real nice premium airline product, you'll usually won't be able to get that for less than a few thousand $/€/£ if paying.
If you travel a lot (especially for work), make sure you are a member of all the relevant loyalty programmes, and try to concentrate your travel on a single one. If you spend a lot, make sure you use credit cards which give you miles. I used to be able to run a lot of business expenses on credit cards which gave me miles and that allowed me to travel in business or first quite a bit.
But this needs significant expenditure, long-haul business or first class flights often require a lot of miles/points (and it seems that constantly increases), and availability is often limited (and now many airlines have several levels of awards: the cheaper ones use less miles but you can only find a seat once in a blue moon, the more expensive ones let you book any available seat but will use sooooo many miles.
You can also use miles to do upgrades, though again, this often becomes more and more difficult: some airlines will let you upgrade from coach to premium coach or premium coach to business or business to first, but not necessarily skip from coach to business or first, and the process can be complex.
There are lots of sites out there which talk about miles and points and free upgrades, you can spend hours (or even days) reading all that, but there's no magic bullet. Airlines reward their better customers, they have no reason to give out seats in premium classes cheaply.