Just my experience using WLP099 Super High Gravity ale yeast. I'm a relatively new home-brewer (less than 10 months), but using this yeast I managed to make a 22% all grain RIS (no sugar/syrup additions). Using a 8L yeast starter, 1 packet of WLP099, 7 stepped additions of wort, 5 lots of aeration using a cheap aquarium pump (atmospheric oxygen, not pure O2), dry beer [pilsner] enzyme and some simple yeast nutrient managed to go from 1.202 to 1.046 and around 74% apparent attenuation.
On big beers this yeast is a slow fermenter, chugging along for quite some time, so be patient as it will get there, just much slower than you expect. As regards taste, as I mainly pitched the yeast cake from my starter (8L out of 15L total starter) I decided to taste the hopped starter beer and it was positively delicious. 90% apparent attenuation (!!), 6.2% ABV, fruity but with little or no weird off flavours/phenolics (although phenols will increase in bigger brews). No sulphur or other homebrew tastes to mention of, despite being still cloudy with yeast (Doesn't flocculate terribly as well as many other strains). Although not my first choice for a low gravity beer, it still tasted great for any English style ale and is unsurpassed for attenuation!
My big beer needs maturing, but 2 months from when fermentation started it tastes like a liquorish, dark fruit liqueur, deceptively smooth with a warm alcohol burn. Surprisingly drinkable already, and no doubt will completely transform with aging. Any yeast flavour is probably masked by the malt as there's no denying the 18kg of malt that went into this beer! However, the yeast did it's job.
Be warned though you can't just toss this in and make anything over 10-11% without stepped additions and simple aeration as it doesn't favor high gravity environments.
Would I make a 20+% all grain beer again? Maybe, maybe note, but time tell (it's alot of work).
Would I use this yeast again? Most definitely. Would suit any super dry English style ale or Barley wine, and possibly even an experimental Belgian-style Trappist