Depends on the particular problem. Rarely will a sealer help a completely blown gasket. That said, the best sealer is Aluma-seal powder. It's cheap 'nuf that if it doesn't work U are not out too much money; just keep your expectations low. Follow directions and add to ONLY a clean system that doesn't have overheating problems, like a partially clogged up radiator.
The problem with these sealers is while they might fix a leak, they seal up other stuff too, namely the heater core and the radiator. It is much better to fix the problem, rather than to cause additional damage with a snake oil product.