100% behind the IN campaign! Admittedly I'm probably biased on the matter as I've seen and experienced the benefits of the EU first hand through my degree. Not saying the EU is perfect by any means (what political organisation is?!) but the benefits of staying in far far outweigh the perceived negatives in my mind.
Wales in particular would certainly be harmed by leaving the EU. I don't think people here quite understand exactly how much money we get from the EU when it comes to grants and funding for projects around the country. We have some of the poorest areas in Europe (something that is just mind boggling for a country in the UK in this day and age...) and the EU gives us a pretty hefty grant to go towards that. I'm not too convinced Westminster will be willing or even able to cough up that type of cash to us!
Same goes for the various improvement / building projects dotted around Wales, like the new Swansea University campus, or the proposed tidal energy project there too. Think I read somewhere that Wales gets around £500m in investment from various parties in the EU, which generates £4bn for our economy. That all will be severely at risk if we leave.
I know this is all centered on Wales, but I believe it's the same across the UK and it's something that's massively glossed over by the Eurosceptics as they go on fearmongering about terrorists and making us "Great Britain" again...
Not sure if people realize the sheer amount of day to day complications that will arise from us leaving too. You can kiss goodbye to the idea of popping over to the continent on a last minute cheap holiday, as you'll likely be needing to apply for a Shengen visa to enter! Trying to cross from Dover to Calais or jump on the Eurostar to Paris will be hell too, with each and every person needing to go through passport and visa checks before having a chance to get going.
Of course I'm sure there's plenty of people who'd welcome that due to wanting to make it hard as possible for migrants to get in. However, ironically, it could potentially make things easier for migrants to illegally try and enter Britain! France essentially regulates our border for us on the French side of the channel by keeping the migrants there (hence the migrant camp at Calais), but if we leave, there's no guarantee France will continue to do that for us. This means we could see a lot more migrants rocking up on our shores than there are now.
I get pretty frustrated at anti-EU politicians going on about how "our current open border is dangerous", when the truth is, we do not even have an open border with the EU! Sure you can get into the UK freely if you're an EU citizen, but you still need to show your valid passport to the proper authority before being able to do that!
Add to that the complications such as what happens to all those Brits living in countries like Spain? They're gonna have to become Spanish, get proper visa's or face being kicked out of the country. I'm also pretty sure that the Good Friday Agreement will be affected in some way by Brexit, kicking that whole hornets nest once again...
I'm praying we do stay in the EU anyway, I'm a big fan of the benefits I get from being an EU citizen, and really don't fancy giving them up any time soon!