I'm currently using GrabIt and I've a feeling it makes most files corrupt or missing.
I never had much problems, but nowadays it seems almost any download can't be fixed.

I blame your usenet provider and not the client. Anyway, I'm using 2: hellanzb on Linux and alt.binz 0.28.2 on windows. Both automatically download PARs and repair when necessary.

My provider is http://www.news.astraweb.com/ The retention is 160 days and I've even downloaded 140 days old untouched Blurays and not a single PAR was needed.

I've found a nice feature in SABnzbd+ that's not available on the other clientes I tried. It's a switch called:

Fail on yEnc crc Errors
Use backup servers on yEnc crc errors.

If an article is missing/corrupt when downloading from your 'main' server, it tries to download those articles from the backup servers. And some articles that were missing from my main server were available on the backup servers.