The popular logic seems to be that the Vita-Saw may be alright. The benefit is obvious, you get to keep your uber at 20% when you die, saving yourself 10 or so seconds for your next uber.
The disadvantage is the point of debate. Most people seem to think that -10 HP isn't very significant. After all, people say, how many times do you survive with 10 HP or less?
I think this argument is self-evidently false. It essentially claims that 10 HP or less is equal to death. In other words, once you taken the first 140 damage, the last 10 damage is inevitable. However, this same logic can be applied to the Vita-Saw. If 10 HP inevitably results in death, then you're dead after taking only 130 damage.
In the end, -10 HP is significant, regardless of armchair generalizations.