Here's my experience: I have a 30L and a 60L HDPE plastic barrel (that's about 8 and 16 gallons). The 30L one I've used quite often, but the 60L barrel not so much (50L is a lot of kombucha to handle). They have quite a few advantages:
* spigot at the bottom
Especially the 30L I find very convenient for brewing a large (25L) batch. The kombucha takes a bit longer to brew than in a smaller batch, but it comes out quite nicely. However, I've never been a 100 percent convinced that this plastic is really safe for making kombucha. Yes, it's supposed to be food grade and safe, and I've never noticed any negative side effects, but how do I know that really nothing is leaking into the kombucha?
I would prefer to brew in glass, but for large batches this is not convenient. Recently, I've bought a 40L ceramic sourkraut crock. It's really heavy and becomes impossible to handle when filled, but at least I'm confident that it's safe...