I just wanted to chip in on two things here - the first is someone upthread asked what industries would be affected by a withdrawal from the EU.
I'm a lawyer advising on corporate, environmental and product regulation laws. These things are harmonised across the EU because most regulation is only of any use if neighboring countries align their requirements together. It's no good cracking down on air emissions here if you can move just across the border and emit huge amounts of nasties into the air that drift across, or waste if you can just truck across the border and dump shit into the river that goes back into your country. We in the UK are actually leaders in trying to pressure other countries to tighten most types of regulations - if we left we would be subject to whatever our neighbours decided to do, and merely being an island will not insulate us from that.
In addition I spend a lot of my time advising companies from non-EU jurisdictions, including countries like Norway, how they can comply with EU laws, which they have absolutely no control over but have to comply with anyway if they want access to the market. As you can imagine this costs them a huge amount of money and they have no say in the content of those regulations but have to follow them anyway if they want access to the market. Again the UK currently takes a leadership role in drawing up these regulations but if we withdrew from the EU we would be subject to all the regulations but would have no say in their reasonableness, scope and effect on our own industries. As the vast majority of our trade is with the EU and many multinationals manage their regulatory compliance work in the UK, leaving the EU would be a hammer blow as we would continue to have to comply but would have absolutely no influence over what our regulatory obligations are. This will massively increase costs for UK businesses, UK-based importers and and drive multinationals elsewhere.
Finally, I would note that having lived in the UK since 1999 as a non-EU citizen and then becoming a British citizen, it has massively increased my personal options and flexibility in terms of jobs. I was offered a secondment to Frankfurt last year - had to turn it down because of my visa. When I was thinking about taking a sabbatical I couldn't stay out of the country for more than 90 days or I would lose my visa. I wouldn't have been able to retire to France, spend more than a certain number of days in any EU country, and certainly I have not been eligible to claim any benefits except the NHS for which I pay at the highest tax rate (and would be willing to pay more, it's FAB). The equivalent would be the position for any UK citizen living and working in an EU country if we withdrew from the EU.
Whatever the result of a referendum and whatever your opinion of spending plans in Brussels, as far as I am concerned as a lawyer working in this area, the UK has been part of the EU for too long and does far too much trade with other EU member states to disentangle itself. The practical result would be that whether or not on paper we are part of the EU, our companies would have to continue to comply with EU regulation in all these areas but our government would lose any ability to influence or control the future development of these regulations. End result would be loss of influence but with no loss of regulatory compliance burden and therefore massively increased uncertainty and costs for businesses.