Using it as a preventive maintenance item will yield results in the long run. While a lot of oils have excellent cleaning properties, such as Mobil 1 0W-40 mentioned by Trav, and do a great job keeping engines clean over long drain intervals, I use Pro-Line Engine Flush as more of a "rinse" prior to an oil change, particularly with extended drains.
The primary elements in Pro-Line Engine Flush are petroleum ether, calcium salt, calcium-based alkylaryl sulfonate detergent, benzenesulfonic acid, PEA and non-ionic surfactant. They also have a "proprietary" agent in there, but there's no detail on what exactly it comprises of. It does a pretty good job, and works well unsticking piston rings from my experience.
In one particular application, a Mitsubishi 3.5L DOHC V6 (6G74 series), there was an issue with oil coming out of the PCV valve and the dipstick not fully seating after the engine's been running. The oil also reeked of gas and would turn dark very quickly. I ran Pro-Line Engine Flush for 25 minutes and repeated the process three times after 500 km (313 miles) intervals using Pennzoil LongLife Gold 15W-40. After the third interval, the oil retained more of a golden color for 2,500 km (1,563 miles) before it started to darken, and the amount of oil coming out of the PCV valve was significantly reduced. The dipstick was seating properly, and once the PCV valve was replaced, oil consumption was no longer a big issue. Proper to replacement, it would consume about a liter every 1,500 km (938 miles) and went to no make up oil between 5,000 km (3,000 mile) drains. The engine's still running great to this day, and the owner continues to use Pro-Line Engine Flush every second oil change.
Long story short, some products do have a place, and this is definitely one of them. Of course, there are various other products and methods you can use.