These are the current possible solutions from this post so far and a few others I found for anyone else looking:
SpreadsheetGear.Net
(Didn't evaluate due to high purchase cost)
Aspose.Cells
(Evaluated by a collegue. Appeared to be fairly simple to implement, performance comparable to Excel Interop).
GemBox
(Didn't evaluate)
Excel Services
(Seems only to be included in SharePoint 2007)
Excel Mapper
(Didn't evaluate because it requires strongly typed objects to import into which didn't fit my requirement).
SmartXls
(Didn't evaluate because it requires strongly typed objects to import into which didn't fit my requirement).
ActiveXls
(Fairly easy to use, lack of Properties raises questions, they have a preference of Methods for trivial actions. Despite it's claim of 1M records a second was out performed by cheaper FlexCel. Have decided that the help/API manual is almost useless.)
Koogra
(Didn't evaluate due to finding no documentations/information)
FileHelpers
(Didn't evaluate)
Flexcel
(Lowest cost solution found, good performance and was simple to implement with a close proximity to Excel Interop structure. Also received quick response to technical question from support. Probably my pick of the bunch.)
SyncFusion BackOffice
(Medium cost and had a reasonable structure. Unfortunately had more difficulty implementing and inconsistent results when running unit tests. Also received a number of 'Attempted to read protected memory' errors, which didn't encourage me with purely managed library.)