For me, first and foremost it's trade and business.
EU countries invest $1.2 trillion in the UK; there are no barriers to investment from elsewhere in the EU. There would be if Britain was not part of the EU or at the least part of the European Free Trade Association (EU plus Norway, Switzerland, Iceland and Liechtenstein - more on this later).
Other member states are forbidden from putting import duties on our goods and services.
We benefit from the heft of a trading bloc worth $16 trillion which is particularly important when negotiating reciprocal trade agreements with other countries/blocs - the UK alone is nowhere near as desirable a market and we would not necessarily get anywhere near as favourable terms.
Our net membership costs are 0.4% of GDP, the CBI estimates the benefits to be worth 4-5% of GDP. (All these figures come from the CBI, by the way.)
There are benefits to individual businesses through being part of supply chains - and having access to supply chains - without trade barriers. In 2009 (not a great year for business), 209 billion of the 293 billion we exported to other EU countries was used by other industries rather than being consumed directly by citizens.
So, why not leave the EU and create a free trade agreement with the EU like Norway, Swizterland and Iceland?
Well, you need to get a look at the conditions of such an agreement and joining EFTA. Membership of this free trade area - the only one realistically open to us if we left the EU - requires signing up to the four freedoms that form the basis of the EU. That is the free movement of goods, services, capital and persons. That fourth is key. Were we to emulate Norway, Iceland or Switzerland we would have to sign up to essentially the same immigration system as we have now.
So the issues people are worried about - in particular, immigration. Yes, it's an issue that's worrying many people, and the pressure on services and issues of integration in particular need urgent attention. Is leaving the EU the answer? I don't believe it is. During the recession, the UK economy contracted by 6.4%. If EU membership is worth 4-5% of our GDP lost, permanently (or even temporarily), that is a lot of businesses, a lot of jobs, and a lot of austerity. It would be 2008-2011 all over again.
My answer is this. Let's recognise that, while paying down the national debt and altering the structural deficit (which is the real issue, not the debt), let's invest in services. Let's make sure that we have a system that encourages integration (selection by religion in state schools is a really significant issue that needs addressing as it does support segregation of ethnic communities). Let's build more schools, fund GP surgeries better and ensure better access to health care, let's make it easy for people to integrate, and for god's sake let's introduce a minimum wage that is the same as a living wage.
And in the medium term, let's work to make the EU work better than it does now, improve the democratic process and make the most of this particular moment in the EU's history to achieve useful change in the way it operates, rather than treating a sore finger by amputating an arm.