I recently finished a POC where we had to show the prospect how we would handle the hundreds of Account related fields they require. Since all of the fields where not required by every Account type (i.e. Account Subtype A had 50 unique fields, Account Subtype B had 75 unique fields...) I decided to use extension tables.
If you follow this 2 step process extension tables will work like a charm:
1) In Application Architect use the New Entity wizard. On the Relationship tab select 1:1 and then select the entity you want to extend (in my POC I wanted to extend the Account entity).
2) When building the data entry form for the extension table make sure to create it under the entity you are trying to extend (e.g. Account) instead of under the extension table (e.g. AcctSubTypeA).
Note, the one easy mistake to make is to create the data entry form under the extension table (e.g. AcctSubTypeA) in Application Architect. If you do this, expect to see an error message like, "ACCOUNT" in the web client when trying to save your changes. The codeless fix is to simply rebuild the form under the entity you are extending (e.g. Account).