I prefer opening a PDF in a tab until I've had a chance to see if it's something I want to keep. With download-and-open-in-Preview I'm filling up my Downloaded Files folder with a lot of PDFs that I don't want to keep, and I'll have to delete them manually.
As a linux user I am surprised. When I download to open in my reader the pdfs go into /tmp. This is fine, because since /tmp is a ramfs it just goes away when I turn off my computer. If I want it I just save it to a permanent location.
Unlike m_eiman and cemerick, I regard dumping debris into my Downloads folder as a feature. I basically never clean it out; it's just a convenience place to keep stuff that I have a low probability of ever wanting again, auto-sorted by how likely I am to want it (most recent first, in other words). Since I almost never bother with more than the first few items in the stack display of Downloads, it didn't occur to me that cleaning it out might be a chore, or even desirable.
Whatever engine Apple is using to view PDFs in Preview, it doesn't always display PDFs correctly.
It's much faster than the bloated beast that is Acrobat, but I'd say about 20% of the documents I open in Preview have to be viewed in Acrobat to be usable, so I've just defaulted to avoiding Preview instead.
A good, fast PDF viewer in the browser would be far better than either of those options.
What kinds of PDFs are you opening? I find the failure rate in Preview is more like 0.5%, and it's generally related to annoying (Adobe proprietary?) form-related stuff.
Plus Preview can also do some very handy basic editing (cropping, adding/deleting/reordering pages, etc.), which I can only assume Adobe have left out to in order to up-sell you Acrobat.
