I needed a LOT of yeast for my bock 2 weekends ago and I built 2 lager starters; one using a White Labs vial and the other using a Wyeast "smack pack". The White Labs vial took off VERY quickly (high vitality) and the Wyeast smack pack took a long time to even swell the bag and then the starter fermentation was very lackluster.
-I ended with WAY, WAY, WAY more total cell count from the White Labs vial.
In addition to getting 5x the cell count, I also find I get better vitality from the vials. (The "best used before" date is only 6 months from the date of manufacture on the White Labs vials.)
I can't say enough good stuff about that product. We get the strain diversity of liquid yeast with the ease of use of dry yeast: "just pitch directly to wort".
Adam