CSV

Pros:

  • Simple

Cons:

  • It may not work in other locales or in different Excel configurations (i.e. List separator)
  • Can't apply formatting, formulas, etc

HTML

Pros:

  • Still pretty Simple
  • Supports simple formating and formulas

Cons:

  • You have to name the file as xls and Excel may warn you about opening a non native Excel file
  • One worksheet per workbook

OpenXML (Office 2007 .XLSX)

Pros:

  • Native Excel format
  • Supports all Excel features
  • Do not require an install copy of Excel
  • Can generate Pivot tables
  • Can be generated using open source project EPPlus

Cons:

  • Limited compatibility outside Excel 2007 (shouldn't be a problem nowadays)
  • Complicated unless you're using a third party component

SpreadSheetML (open format XML)

Pros:

  • Simple compared to native Excel formats
  • Supports most Excel features: formating, styles, formulas, multiple sheets per workbook
  • Excel does not need to be installed to use it
  • No third party libraries needed - just write out your xml
  • Documents can be opened by Excel XP/2003/2007

Cons:

  • Lack of good documentation
  • Not supported in older versions of Excel (pre-2000)
  • Write-only, in that once you open it and make changes from Excel it's converted to native Excel.

XLS (generated by third party component)

Pros:

  • Generate native Excel file with all the formating, formulas, etc.

Cons:

  • Cost money
  • Add dependencies

COM Interop

Pros:

  • Uses native Microsoft libraries
  • Read support for native documents

Cons:

  • Very slow
  • Dependency/version matching issues
  • Concurrency/data integrity issues for web use when reading
  • Very slow
  • Scaling issues for web use (different from concurrency): need to create many instances of heavy Excel app on the server
  • Requires Windows
  • Did I mention that it's slow?