My company has begun to work with Covast's EDI Accelerator for BizTalk Server 2004 and I've got some definite initial impressions.
Pros : Definitely capable of doing what it is advertised to do. Makes controlling batch numbers and other such things considerably easier.
Cons : In a world of XML based messaging, and integrating with BizTalk Server as this product does, it still uses delimited flat files under the covers to describe it's messaging structures. It just seems out of date.
I'll post more as my experience grows and as I do more work with the 270 and 271 transactions.