More experienced folks here have said it all. I'll just add: I'm using a CK20HST on a couple of acres, do all sorts of loader and BH work, but no mowing. I'm too tall (6'4") to be comfortable in either seat, and my right foot gets tired on the pedal from the awkward angle. Mowing or doing other long/steady runs with the HST would be a chore. But for general landscaping, I can't imagine getting so much done with all the shifting. And for plowing snow close to house and car, it's a real gift.
I was dubious about the cost $1500, I think?), but decided in might pay off in resale value. As it worked out, I admire almost everything about this tractor (not counting a persistent drip in back which the dealer couldn't/wouldn't fix, and that missing inch or two of foot room even with the seats back), and am *very* glad I bought the HST.
-Chelydra
P.S. - I worried about the loss of power. I usually run at about 2400 RPM for loading, 2000 for BH work. Have only run up to PTO speed or full throttle to get out of scary holes or side-hill situations. Most other times, power is limited by hydraulic blow-off, so I run at a speed that gives comfortable movement speeds - faster for the loader than the BH.