George Will is very critical of the Seventeenth Amendment. Publius at "Obsidian Wings" defends it with the standard anti-corruption line. This argument has always seemed to me to be rather weak, however; the Amendment itself does very little to curtail corruption. Rather, I would suggest what it does -- and what it primarily was intended to do -- is curtail the effects of incompetence.
While corruption was an issue, and there were several notorious bribery cases, the underlying reason for the amendment is not that legislators are often bad, but that legislatures are often incompetent. Intense partisan conflict in state legislatures had begun repeatedly creating deadlocks, so that states were regularly ending up without full senatorial representation. If I recall correctly, Delaware at one point went for several years without any Senator at all. Such a form of representation is problematic, unstable, and probably unsustainable -- even where there is no corruption at all. The Seventeenth Amendment arguably does not do much to handle the corruption problem, except very indirectly: governors may be as corrupt as legislators, and the only compensating factor is that it is slightly easier for a legislature to do something about a corrupt executive, if clearly corrupt, than for anyone to do anything about a corrupt legislature even if it is clear that it is corrupt. But it does handle the incompetence problem very nicely: Senate seats get filled quite quickly and efficiently. It is not so clear, I think, that they get filled with less corruption; but they do get filled, and that is a big plus.