First off, putting extra cash toward a mortgage early on, when most of the payments are going to interest, is the BEST time. If you pay an extra $1 on your mortgage today, you will save 30 years worth of interest (assuming a 30 year mortgage). If in 29 years you pay an extra dollar, you will only save 1 year worth of interest.
That said, there are lots of things that go into a decision like this. Do you have other debts? How stable is your income? What is the interest rate on your mortgage compared to any other debts you may have or potential investments you might make? How much risk are you willing to take? Etc.
Mortgages tend to be very low interest, and, at least in the U.S., the interest on them is tax-deductible, making the effective interest rate even lower. If you have some other loan, you are almost always better to pay the other loan off first.
If you don't mind a little risk, you are usually better off to invest your money rather than pay off the mortgage. Suppose your mortgage is 5%. The average return on the stock market is something like 7% (according to my buddy who works for Wells Fargo). So if you put $1000 toward your mortgage, you'd save $50 the first year. (Ignoring compounding for simplicity, changes the exact numbers but not the basic idea.) If you put that same $1000 in the stock market, than if it's a typical year you'd make $70. You could put $50 of that toward paying the interest on your mortgage and you'd have $20 left to go on a wild spending spree. The catch is that the interest on a mortgage is fixed, while the return on an investment is highly variable. In an AVERAGE year the stock market might return 7%, but this year it might return 20% or it might lose 10% or a wide range of other possible numbers. (Well, you might have a variable rate mortgage, but there are still usually some defined limits on how much it can vary.)