How do you work out if it's worth getting an jnterrail ticket? Does anyone have experiences to share?

What do you mean with ´worth it´ If it would be cheaper? We do not do the math-you have to check yourself if booking a row of normal advance tickets would work out cheaper. The prob is that on a pass you will need to pay for expensive extra seatREServ. on €* and maybe for the Kobenhavn-Sweden train. To LON and from BRU it is not mandatory-but you run the terrible risk of not being able to find a free 4-seater for your family. Best advice for booking is -as always-on seat61.com, it will also describe best way of choosing trains. It may also be that the child has to pay (discount) for some sections on advances.

MIN for an IR-pass is 4 days-you can quite easily do this single trip in 2 days-with a break in f.e. Hamburg or around. This month passes are 10% discounted-start using can be later.

We start with the math :) 

Check each leg and compare the conditions cheap advance fares are often not exchangable or/and refundable. 

You may split the Journeys ;) 

Cardiff - London - Brüssels (note that eurostar neeed a reservation on top of the railpass and a seat is per person arround 35€ :/ ) From Brussels to Hamburg you can use the reservations of german rail for 8€ in 2nd class or 10.60€ in 1st class for 2adults and up to 3 kids/youths (the reservation is optional but highly recomened if you wanna sit together) 

Hamburg - Copenhagen is usally reservation compulsory in summer adds another 4€ per person 
From copenhagen you have several trains to Gothenburg :) 


Between Kiel and gotheburg operate a daily ferry that offer a small discount for Railpassuser (maybe it worth it to travel from Brussels via cologne/Hamburg to kiel and wake  up the next morning in gothenburg by using the  ferry.