We've integrated with the Sage and SAP ERP systems as well as Quickbooks with Sugar 7 using Starfish ETL: http://starfishetl.com/. Starfish works well but has a bit of a learning curve. Any issues we've found have been with the API of the target product.
May I suggest you check out Commercient's integration app. They have implemented over 85 integrations between multiple ERP & CRM's Including Quickbooks, Sage, Oracle to name a few. Their integrations are highly customizable and can be as in-depth or as simple as needed. The app is also available on SugarExchange.
