is the gog.com downloader pretty reliable (i.e. no corrupt files, pausing and resuming, etc.)?
I think it's only been out a few days, so I'm sure it has a few kinks to work out. But I've tried it and it seems okish. It supports pause and resume. It lets you choose where to place the downloaded installer, and seems to make a new folder per game. It's now a small, self-contained, native win32 program (the old GOG downloader was an Adobe Air app).
File corruption on downloads is more a sign of failing hardware, and all a downloader can really do about that is perform checksums of downloads and automatically redownload if a file is corrupt. The downloader claims to be doing an "integrity check" at the end of the download, but I'm not sure how fine-grained the checksuming is (to allow for finer-grained redownloading), or what action it takes if the integrity check fails (automatic download, warning dialog, etc).
It seems to be a multi-part/multi-connection downloader, so it gets very fast download speeds. Unfortunately it seems to save into multiple 10MB "chunk" files and then copy everything into a single file at the end, which will be bad for disk fragmentation (most multi-part downloaders allocate a single full-size file at the beginning to avoid this).
After the download, it can save you a few clicks in Windows explorer by launching the installer for you, and it can also handle deleting the installer from disk after. Any of the extras you want still have to be downloaded "manually" through a browser, as far as I can tell.
Honestly, you don't gain too much over just using the website, except for the speed of the multi-part downloading (since they seem to have changed their server behaviour to no longer allow third-party multi-part downloaders), and fewer clicks when downloading a game large enough to be split into multiple downloads (GOG splits downloads up if they're bigger than 2GB).