Depends how you plan your journey, i done ljubljana in Slovenia to cologne in one day using one of my inter rail days.I think it was 800 miles travelled that day, i went via nuremberg and just changed trains a few times to reach Cologne. Most i have done on interrail is 7500 miles in 30 days, so it is fairly cheap if you get that much mileage out of it.As everyone has said the countries you are travelling in is very relevant as to how much the saving will be. Travelling the whole of Germany in one day is likely to cost more than £40.
I remember I got one about 15 years ago, a lot better value back then, you could use it everyday during its duration, if I remember correctly....Although I was broke, manage to travel half Europe within a month...
http://www.seat61.com/news.htm has a comment about recent changes to the rules for using Eurostar tickets if you have an Interrail ticket. You pay €30 extra. You either need a Europe-wide Interrail ticket, or one for France or the Benelux countries. I'm interested in the 3 day in 30 pass for Switzerland for £90
From/to U.K. You can use Eurostar which is not cheap.. plus you can use local buses and other transports.
My thoughts exactly. And Czech Republic trains are cheap, places like Bulgaria the buses are cheaper and better than the trains.
If you're spending a lot of time in Eastern Europe, you might be better off getting busses between your locations - could be cheaper and it often takes as long as the train, possibly more comfortable as well.
Double check how much it will cost you to buy train tickets as you go along before buying these passes. I'm sure they work well on certain routes - particularly if you need complete flexibility - but every time I've looked it's been much cheaper just to buy tickets individually.