I was going to write up a piece on thallium poisoning, until word came out over the long weekend that the Russian spy case was instead an instance of polonium poisoning. That's a very different matter indeed.
For starters, polonium isotopes (like most radioactive substances) are much more hazardous as radiological agents than as chemical ones. Unraveling the two isn't always easy, but this case is pretty clear. It's likely that polonium is chemically toxic, since it's in the same series as selenium and tellurium (which both are), but it's also likely that any reasonable dose would kill a person from alpha radiation rather than from whatever enzyme inhibition, etc., that might also ensue. People have been dosed with fairly robust amounts of tellurium and survived, albeit uncomfortably, but I can't imagine that anyone has been exposed to a systemic dose of a hard alpha emitter and pulled through.
This takes us into the long-standing arguments about the toxicity of such isotopes. Readers who remember the anti-nuke days of the 1970s and 80s may recall the statements about plutonium's incredible toxicity, generally expressed in terms of how miniscule an amount would be needed to kill every human being on the planet. Left unsaid in those calculations was that said plutonium would have to be dosed intermally in some bioavailable form. More Pu was surely vaporized in the atmospheric bomb tests of the 1950s, without depopulating the Earth to any noticeable extent. (See the arguments here, for example).
Here, though, we have a case of that exact bioavailable dosing of a strong radioisotope, with the unfortunate effects that you'd predict. There were some experiments early in the atomic research era where patients were dosed with radioactive isotopes. Oddly, the polonium experiments may have been the only ones that stand up to ethical scrutiny. A good review of what's known about polonium exposure, at least as of 1988, can be found here.
One thing that many people may not realize is that every person on the planet has some polonium exposure. There are many people who equate "radioactive" with "man-made", but those categories don't completely overlap. Polonium is a naturally occurring element, although certainly not in high abundance, but there's enough for Marie Curie to have isolated it. It's part of the radioactive decay series of U-238, and as a daughter radionuclide is a contributor to the toxicity of radium and radon exposure. You've had it - but not like this.