A sinking fund provides credit enhancement for the investor and reduces interest rate risk for the issuer. The obligor will purchase a pre-determined amount of bonds to be retired in the open market if they're trading below par, or they will make payments to the appointed trustee who will buy-back the bonds in a lottery(typically at a pre-determined call price). Schedules vary considerably and may include other provisions such as deferment periods or acceleration features. One just needs to read the indenture to see how predictable the schedule will be.
From the investors point of view, the advantages of a sinking fund is that it ensures a timely paydown of the principal so the maturity payment is easier to make. It will enhance the liquidity of the debt, which is especially nice for smaller issues in thinner secondary markets. Finally, the pricing will be more stable since the issuer may become an active participant in the buy side when prices fall. All of these will nudge the bond towards lower yields and tighter spreads.
There are some drawbacks, especially if you happen to be one of the lottery bonds called early, wasting your time in analyzing the bond and potentially relinquishing a high coupon if rates fall. This can put downward pressure on new issues prices when interest rates are high since contraction risk will be greater. Other features such as acceleration can also put downward pressure on prices.
As for actually pricing the bond, treat it as you would a callable bond. The indenture, or preferably your data provider, will give you the information you need. Besides the obvious coupon/frequency etc., you need to know the probability of it being retired(amount to retire/remaining issue size), the price you will receive if retired, and the corresponding schedule.