Some choice quotes:
"Yes, using SDE effectively castrates the spatial database. It still walks and talks, but it's a shell of the man it was before. " – yours trulyI'm thinking of becoming an instigator. Apparently there's money in p***ing people off.
"The 'economical' route for those that want to use PostGIS and have edit capabilities inside ArcGIS desktop would be to use the ZigGIS professional which (from my understanding) implements an editable PostGIS layer in ArcGIS desktop." – Rob Tester
"If you compare [pricing] with Oracle then yes probably so... But honestly if you are talking about SQL Server vs. PostgreSQL, I think the savings you get from running PostgreSQL would be dwarfed by the cost of SDE." – Regina Obe
"Spatial databases are being commoditized, but SDE is not a spatial database, it's a revenue enhancer for ArcMap, and ArcMap is *not* being commoditized." – yours truly
"A few disclaimers and background : Until last year, I worked at ESRI. I contributed with some of bug fixes for the PostgreSQL/PostGIS support in the ArcObjects side of things." – Ragi Burhum (read this one!)
"There is one thing to note in a mixed-client environment and that is this: if a non-ESRI client writes an invalid geometry to a the database then when ArcSDE constructs a query over an area that includes this feature ArcSDE will discover the error (as it passed all queried features into its own shape checking routines) and stop the query" – Simon Greener (read this one!)