The condenser in my car's air conditioner is leaking. A mechanic said that he could try to seal it. I was wondering if it is worth sealing, or should I just replace the condenser?

As long as the fix is fairly inexpensive allow him to try and seal the leak. It does not always work but when it does it significantly reduces the price to have it fixed that way over the price of purchasing a new compressor. If the mechanic offered that to you as an option then I would assume he feels fairly certain that it is a small enough leak that he will be able to seal it.

Dd:

You used the word "compressor". Did you not mean "condensor"?

OP:

There are SIGNIFICANT costs involved in changing a condenser in a car. If your willing to spend about 8-12 hours changing it, I think you can. Just:

1. Have certified installer remove refrigerent
2. You will have to learn how to put the lines together. You will need some compatible refrigerent oil for the "O" rings.
3. You can remove and replace condenser. Remove it and install it, but don't put all the covers back on until leaked checked.
4. Take to installer to leak check and charge.

In a lot of cases the hole is created by leaves and water especially if you park near trees.
I have a fix for that too.

KISS:
The OP didn't list their type of vehicle but it's very rare for a condenser to take 8-12 hours. It's usually located by the radiator.

Allison:
You can try to seal the condenser. The problem is that the system has been opened which means there could be contaminants in the system(i.e.: water, dirt, etc.) If you opt to have the condenser replaced, you will also need to replace the accumulator(also called a receiver dryer) and the orifice tube, if applicable for your vehicle. These 2 items are filters for the A/C system and it's recommended to replace them any time the system has been opened.

This:

Oops: I was thinking evaporator in the passenger compartment. That is a chore.

The condenser is a much easier part to replace.