Whether it is worth depends on your itinery. If you have not planned well in advance, want to travel by random train selection, stay longer at places which you like, and depart earlier for places which disappoint you, your pass may be o.k.
If you would stick to certain trains chosen and book individual tickets early enough you may be cheaper off by individual tickets, on the other side just imagine you've learned to know some cool people and must depart because of your ticket is only valid for a certain train.
You will have to pay for mandatory reservation if you use national or international highspeed or nights trains. Furthermore the Eurail pass is not valid in Great Britain and you have to pay if you plan to go there.
I have attached the website of Interrail which is the European rail system for young European people but the surcharges listed are the same. For journey planning I recommend the use of the German railway website. It is easy to handle and contains all European train connections.
Source(s):
http://www.interrailnet.com/2_special_trains
http://www.interrailnet.com/2_reservations_international_trains
http://www.interrailnet.com/2_night_trains
http://reiseauskunft.bahn.de/bin/query.exe/en