How reliable is the adler32 checksum?

I wonder how reliable the adler32 checksum is, compared to e.g. md5 checksums? It was told on wikipedia that adler32 is "much less reliable" than md5, so I wonder how much, and in which way?

More specifically, I'm wondering if it is reliable enough as a consistency check for long-time archiving of (tar) files of size 20GB+?