An editor from a leading IT magazine asked me this question just now, so I thought I'd also blog about it. Here it goes:

Q1: What are the capabilities of your services organization to help clients not just with implementing your BI tool, but with their overall BI strategy?

The reason I ask this as a top question, is that most BI vendors these days have modern, scalable, function rich, robust BI tools. So a real challenge today is not with the tools, but with governance, integration, support, organizational structures, processes, etc – something that only experienced consultants can help with.
Q2:  Do you provide all components necessary for an end to end BI environment (data integration, data cleansing, data warehousing, performance management, portals, etc in addition to reports, queries, OLAP and dashboards)?
If a vendor does not you'll have to integrate these components from multiple vendors.
Q3: Within the top layer of BI, do you provide all components necessary for reporting, querying and analysis such as report writer, query builder, OLAP engine, dashboard/data visualization tool, real time reporting/analysis, text analytics, BI workspace/sandbox, advanced analytics, ability to analyze data without a data model (usually associate with in-memory engines)?
If a vendor does not, believe me the users will ask for them sooner or later, so you'll have to integrate these components from multiple vendors.
I also strongly recommended that the editor discounts questions that vendors and other analysts may provide like “do you have modern architecture”, “do you use SOA”, “can you enable end user self service”, “is your BI app user friendly” – because these are all mostly a commodity these days.

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