I also like the fact that if anything happens to a walkie talkie, it's a good bit cheaper to replace! I don't have a problem letting my kids use the Motorolas we have... but my cell phone, ummm, no.
Also too -- CHECK YOUR PHONE POLICY. Many cell phones have roaming fees or cost more minutes, etc if outside designated calling area. Could be painful to find that out after the fact.
I will say, however, that the nice thing about the cell phone is if your child is lost -- you have an immediate source of contact. We take our cell phones with us and give each kid a small tag (like a key ring -- attaches to a belt loop, sneaker lace, etc) that has our cell # on it. If we ever lost a kid they know they can have a CM contact us. With the radio systems, you never know if the frequency you choose is going to be the favorite of other folks as well. We follow the rule about radio frequencies (use family birthdays... unusual numbers that everyone can remember! Like my birthday -- Oct 6 -- would be Channel 10, selection #6.) We know that if we can't reach the other radios using son #1's birthday, we then try birthday of son#2, then dd's. Never had to go past son#2 yet!