As stated in the very first reply, it's entirely dependant on the campaign. A campaign that features undead will make Turn Undead a useful ability. If you play undead straight out of the book, they'll be useful in a number of encounters. Thegogmagog did a good job of showing a few example encounters. I disagree with him slightly, though. In an encounter with, say, 3-5 undead, turning 1-2 of them is a significant advantage. That takes them out of the fight for several rounds. If they can't get away even better, as they'll merely crouch in the corner cowering until you get around to whacking them.
Even single undead "boss" monsters can sometimes be turned. As a player, I once had a PC that had the equivalent of Paladin turning ability (back in 3e when it was level-2, not level-3 as it is in 3.5). With a high charisma and a fantastic roll, she managed to turn a lich once. The look on the DM's face was priceless, and it's one of my fondest D&D memories. (Not that the lich didn't come back 1d10 days later.

)
In a campaign that doesn't have many undead to fight, or wherein the DM always makes sure to use unturnable undead (single critters of appropriate CR with massive numbers of HD, for instance,) naturally the ability will be less useful.
As a side note, it's always a good idea, IMO, for the cleric to pick up at least one Divine Feat, so that when not fighting Undead, his turning attempts can be put to some use. Even in Undead-heavy campaigns they're unlikely to be the foe in
every encounter, and as a renewable resource, not using turn attempts is simply a waste.