For me, the issue is not about immigration.
What concerns me is:
When the chips are down, the countries in the EU seem prepared to drop one another in it in order to protect their own interests. I know that there are other powerful power blocs such as the US, Russia and China, and perhaps it's inevitable that we have to join some sort of "club" for economic protection but let's not kid ourselves that the EU is in any way a benevolent organisation.
The way that Greece has been treated by its EU "friends" has appalled me. Firstly, it has been subjected to the most brutal austerity programme that financial commentators say has actually further weakened its economy. Refugees and other migrants have been shunted from one place to another and treated disgracefully by some EU countries and Greece, already in a terrible financial position, was left to cope with the rising number of people arriving on its shores. (We don't, incidentally, hear very much these days about refugees drowning in the Med even though there have been several recent incidents of 300 or more people drowning).
There is also some question as to whether the EU is a stable and sustainable organisation. Since 2014 the ECB has been pumping money into the EU and, together with the current European Quantitive Easing Programme, started in March 2015 and which is supposed to continue until at least September 2016, 1.1. trillion Euros has been injected into the EU economy.
Worse still, there's the issue of the EU entering into top secret TTIP negotiations with the US, which will give multinationals the authority to sue governments in secret tribunals for "loss of profits" if they dare to, for instance, place restrictions on certain products or services - e.g. bring in plain packaging for cigarettes or to open up public services to privatisation. Perhaps this is why Obama is so eager for us to remain within the EU (though the Stay campaign argue that we would be forced into entering a similar sort of agreement if we were outside the EU - and would have less negotiating "clout").
On the other hand, I fear very much what will happen with regard to environmental, employment, health and safety and consumer regulations and rights if we leave the EU. I don't like most of the people that support the Leave campaign - for the Conservative: Iain Duncan Smith, Boris Johnson, Michael Gove, et al, or Kate Hoey for Labour, Marin le Pen, Nigel Farage, etc. I also suspect it is true what Deborah Orr said in the Guardian last month:
"Trump and the Tory Brexiters are cut from the same cloth. They’re anti an establishment that limits and contains them. They want to be at the centre of an establishment in which they are free to do whatever they please."
At the moment I see it as being between the devil and the deep blue sea.