Herolab is certainly the best program to use for
Pathfinder RPG, either as a player or especially as a GM; however, the downside is cost.
Herolab is free to try out, but you can not save or print your character or monster with the program in demo mode.
I would point out to others that are reading this that
Herolab will be offering in early November a special pricing scheme for the Bestiary add-ons. Currently, the
Bestiary add-ons are priced at $15 each for
Bestiary 1 and 2. Those who wait for the release of
Bestiary 3 in early November will be able to purchase
Bestiary 1, 2 and 3 together for $30. That's a significant savings.
I really can't say enough about
Herolab. It is, far and away, the most important toolkit for character creation and monster/encounter design for
Pathfinder. The only downside to it is cost. For regular
Pathfinder GMs,
the cost is worth it. For those engaged in developing a one-shot, however, I think the cost is too high. You'll be happier with a book and a sheet of paper and a pencil, working out your modified monsters by hand.
Frankly, I would recommend all
Pathfinder GMs try this by hand a few times even if they have a full blown install of
Herolab with all add-ons as it will help a GM to understand how advancement and templating actually works.
Obviously, I'm posting this response as much for other people reading this as I am in responding to the OP.
One last comment: Colen McAllister, lead developer at Lone Wolf on the
Herolab line is starting a regular segment on
Herolab and will be a regular recurring guest on each episode of
Chronicles: Pathfinder Podcast beginning with Episode 016 (should be out next weekend). So if you have any questions/complaints on working with
Herolab -- you can send your questions to me or post on our forums and Colen can address your question directly on the podcast.