Do you normally use bisnes class? Do you normally travel 1st?

In Scandinavia about any local train does not have 1st. 2nd is most often better as even 1st in many other countries there-but 1st gives you often access to free cf/OJ and maybe the odd lounge here&there.

Best buy for 1st is by far UK=there you will be treated as a normal bisnis class.

IN DE/DB f.e. passholders are NOT allowed into lounges-but OeBB in AT does.

Is it worth it? Only you can answer that.

What you get in 1st is a bigger seat with better legroom, as a couple you can also get a table for two on most long distance trains so you will not have to share with others. It is likely but not guaranteed you will also get a less crowded and quieter environment in 1st.

I did pay for 1st class ticket, but apparently people are travelling 1st class with 2nd class tickets in Scandinavia. Just book the 1st class seat in Sweden and Denmark. 
 

There were lots of problems with lounge access. In Sweden they were closed weekends, and in Germany I was sneered at and denied access to lounges. No answer from the Interrail company, you can’t do anything if the personnel at the door thinks,your ticket is not valid. They were comparing the 1st class Interrail to the popular 9€ ticket. 

In Austria it should work Ok lounges, but I didn’t test.

In Finland you can’t book seats, so it was convenient to use the almost empty  Extra class. If the train is full in Finland, you’ll just have to find a spot somewhere.

I’m doing two months first class Interrail. For me first class is worth it looking at the amount of travel and the distances I will do in two months. 

Best perks for me

• Significantly better seats with more personal room (typical 2+1 instead of 2+2 on each row). Valuable on long distances. 
• Easier to get seat reservations on quota trains, night trains and normal reserved trains as first class seats/reservations tend to sell out less quickly. Important benefit when travelling flexible without a predefined plan. 
• Easer to get seats on trains without reservations as first class is typically less busy, especially during the summer when we’re typically travelling. 

Additional nice to haves

• complimentary food and drinks on some trains 
• lounge access with complimentary food and drinks in some locations

I've just travelled 1st class from UK to Poland.

It was definitely worth it in UK and on Eurostar but didn't make any difference in Germany or Poland.

We missed our Eurostar connection because of an accident and got on the next train - I don't think this would have happened with a 2nd class ticket as it was so busy.

All seats were great in Germany, all seats were full in PL so had to stand, even with 1st class ticket.