I just got a Piett via trade, and while he's pretty good, he's nothing to pay $20 for IMO.
Now that I have him, I have to say I like him, but many people wouldn't. He's got this walking leg stance- it's not overdone like General Gear Lando or anything, but it does border on preposed a bit more than the other POTF2 Imperial officers do (Tarkin and Thrawn, anyway).
His uniform is in fact a unique sculpt but very similar to Tarkin's in proportions and design- they matched up the tunic bottom a little better but it's still not quite right, I think Thrawn is the only time Kenner/Hasbro got that right while they were still doing "skirts" that way. The paint job is mostly okay but mine has this weird black spot on his left hip- it's scratching off by hand with some work, but it's something to consider.
As for the likeness, he looks a bit like the actor but not as skinny or aged. He looks like perhaps a younger Piett than what we see in ESB and ROTJ. He also doesn't have quite as many wrinkles as he should, I think.
Overall, a good figure, especially among POTF2s, but... it's a POTF2. And it's not really a masterpiece at that.