Binding of Isaac alone offers many hours of frustrating achievement hunting. It took me a long time to become proficient and 100% the game. With Wrath of the Lamb, the difficulty increases ten fold. It brings even more item drops, usables, monsters, and bosses, plus new curses, new rooms, and now trinkets. In fact, there's so much new stuff that I usually have to play with the Binding of Isaac wiki page open just as a reference to remember what a lot of the items do.
My only regret is that you cannot play with the DLC turned off so once you own it, there's no going back. I would recommend new players skip the DLC until deciding they like the original Binding of Isaac and are looking for an additional challenge. This is a must own DLC for seasoned veterans of the original--a fantastic value for the price.