I like to give them some of the worksheets, primarily depreciation schedules and the two-year comparison.  Makes it easier to answer the question of "why was my refund bigger last year than this year?"  "Because your income was up 10% but your withholding was only up 5%".  I think that might be especially useful this year when withholding is all kinds of broken.  In some cases it'll be more like "your income went up by 10% and your withholding dropped by 37%, so get out your checkbook."  I'm not sure the drop from the 90% rule to an 85% rule for 2210 penalties is going to help everyone but it's better than nothing I suppose.  I just glanced at the IRS email on that one so maybe there are more helpful details buried in it somewhere.