On his own, Soul Swallower is a 3/3 Wurm with Trample for 2GG. But once you have Delirium active, at the beginning of each upkeep, you get to put 3 +1/+1 counters on him. Very quickly, he can become very big. Having Trample means your opponent can’t simply chump-block, either. His effect is a bit slow for Constructed, though. Soul Swallower is pretty good in Limited, though, as seen in this winning Shadows Over Innistrad Sealed pool.
There is an Enchantment called Hardened Scales, which gives a creature an extra +1/+1 counter whenever they would gain one. Unfortunately, it was no longer in Standard when Soul Swallower arrived. It's not like such a deck has very good synergy with Delirium anyway. Still, Soul Swallower was a bomb in Limited once you hit Delirium.
Soul Swallower does see some EDH / Commander play in graveyard-based decks like those led by The Gitrog Monster, Sidisi, Brood Tyrant, and Ishkanah, Grafwidow (who has Delirium herself). Soul Swallower is another well-designed creature from the Shadows Over Innistrad set.