On the other hand, with things like software and business model patents, this is disastrous.
1. Some open source developer creates some cool piece of software, algorithm, or whatever.
2. Big Company (tm) see's this and decides it's cool, and files patent.
3. Big Company (tm) sues open source guy (and any other competitors it feels like) over said patent.
4. Open source developer who invented the cool software/algorithm/etc. is forced to abandon his/her own code because or fight in court - probably signing the rights over to Big Company (tm) to avoid being sued (just the cost of defending against a suit is big enough to make most people give up, even with smaller damages).
5. Profit for Big Company (tm) as they re-brand open source developer's code in their own name.
The cost and effort of filing for patents means that a lot of open source developers, and small businesses just won't do it. First to File combined with software patents has the potential to be a nuclear device in the software world - where only the bigger players are protected (by having large patent portfolios that they can use against each other) - the smaller fish are now no longer protected by being able to prove they actually invented their own code first - because they just didn't have the resources to file a patent on it.
Sure, First to File is easier for the patent administration office - but this is one case where 'well everyone ELSE is doing it that way' is not a good excuse for changing the law. There are some things (for example health care) where the US can and should take a page from other country's books (socialized medicine is really not a bad thing - and no, it won't create a nanny state, but your health should be a fundamental right, just like your freedom of speech). The first-to-invent part of patent law is something where the US got it right and the rest of the world didn't, they took the easy route - but not necessarily the better one.
So I say, keep 'prior art' or get rid of software & business model patents (which should never have been allowed in the first place). Losing the former without ditching the latter doesn't bring the US in line with the rest of the world anyway (very few places in the world allow software patents) and the combination is disastrous.