Back when WotC was releasing the so-called "Premium Reprints" of older products, they stopped selling PDFs of the products that were up for reprint, presumably to help boost sales of the print products. The only two 3.5 supplements which got Premium Reprints were the Magic Item Compendium and the Spell Compendium.
Now, three years, later, WotC has resumed sales of the Spell Compendium and Magic Item Compendium as PDFs. The new PDFs are supposedly based on the premium reprints. Previously when they re-released PDFs, people who had previously purchased the PDFs got them re-added to their library automatically, though sometimes with a delay. I don't know if they are doing it this time.
Speaking as someone who has both books in print, I can say that the Magic Item Compendium is a must-have for anyone serious about running D&D 3rd edition or Pathfinder. Aside from a huge collection of magic items, the book presents new optional subsystems for combining and enhancing existing magic items. IMO, it is one of the best supplements I have ever used.
The Spell Compendium is...far less spectacular. It's primarily just a big collection of spells, with some setting-specific names stripped out. While there were supposedly revisions to older spells, in reality very few were changed in more than name, except for 3.0 spells which were updated. The Spell Compendium is also far less "complete" than the MiC, as the SpC was released in 2005 and there are three years worth of supplements that weren't included in it. Some people think it's worth it just to have more spells in one place, others don't. In terms of digital products, it might be worth getting for the searcability that comes with digitization...or not.
