PGI, in probably the best marketing move for Clans so far, has announced that they're going to be updating the PTS client with the Clan pack and letting it run 12 hours uninterrupted. The caveat is that you can only the use the Clan mechs you actually paid for, otherwise you can only fight against them. You can find the details by clicking on the image below:
This is a great chance to get a feeling of the Clan mechs and their impact on the game. Even though I don't own one(being a poor College student), I will be participating with my best mechs to compare them against the Clan equipment.