Using yeast from a local brewery

I think the earlier two answers are great, but one thing which is missing is that it is also potentially important is how well the brewery maintains their yeast and their overall quality control. Depending on the beer you are making and how quickly it will be consumed, I think it is important to understand how well the brewery keeps their yeast. A lot of smaller breweries don't do a great job of this (and to be fair, if they are serving beer on premises (ie. not bottling) and moving their beer quickly, it may not be their most critical issue).

So things I'd look for:

• Do they bottle? You have to maintain higher standards if you are bottling. If they do, have you had an off bottle from them?
• Do they have a lab? Doesn't mean that they know what they are doing, but at least it shows an interest.
• How long do they keep their yeast? Some breweries get fresh yeast from a supplier on a regular basis. Others will get fresh yeast for every batch. Some maintain their own and only bank their strain for emergencies.

These issues are much less important if you are looking at yeast for a pale ale that is going to be consumed at a party as soon as it is ready.

One benefit of getting a jar of yeast slurry is that you won't need to make a starter; there will be plenty of yeast to directly pitch into a batch. Depending on how much they give you, it might last you a couple of batches.

Other than that, Matt's answer is spot on.

What kind of beers do they make? A lot of breweries will have a "house strain" that they use for almost all of their beers. If they do a lot of belgian beers then chances are it's a belgian yeast that creates a lot of fruity esters and spicy phenols. If it's German Ale yeast it's going to have banana and clove like qualities.

Just ask the brewery what style of yeast it is (Belgian, German, English, Saison, American etc....). Ask for attenuation and flocculation values.

There are dozens of different yeast strains out there all with different properties. That doesn't make one more desirable than another. Just that some are suited better for different beers. Saison yeast is great for saisons because they are high attenuating and leave a dry beer. Nottingham yeast is great for English ales because it's clean and accentuates malt.

Without the right knowledge and equipment you can't really make a yeast "evolve" to create the properties you desire. Start by reading Mr. Malty from start to finish. It should answer most of your questions very thoroughly.