Very difficult to answer. I have studied it at length, and I don't know the full pros and cons.
This is where a referendum would be difficult, because you can bet the media will stir it up, give out false information - for both camps.
From what I can understand........
Pros.
Many companies are based in the UK, and give UK jobs, because this puts them within the EU, and they thus have access to the EU market. If say a Japanese car manufacturer was to make cars over there, and ship them to the EU, this would incur tariffs.
The EU buys UK goods, and some say if we were not in the EU, then this market would dry up.
The EU has passed laws which go over UK laws, and which prove beneficial. One law being that of banning junkmail (Seriously - it's against the law to send unsolicited junk. The companies get around this by getting you to tick confusing boxes agreeing to your details to be shared with "selected companies" -this means they will pass your details to junkmail distributors)
Shared interests and allegedly backup. The EU are supposed to be our "friends". Hmmmmm.
Cons.
Billions of pounds given every year to the EU budget, and nowhere near that amount gained back. Some may say this is worth it to protect the jobs of the companies who only locate in the UK because of the EU market.
The EU sells more to the UK than the UK buys from the continent. If they tried putting up trade barriers should the UK pull out..........well LET THEM JUST DARE!!
Decimation of the UK fishing industry. Other EU states seem to simply ignore the rules, especially the likes of Spain, with their huge factory ships hoovering up fish by the tens of tons in what used to be UK waters.
The UK is not allowed to be self sufficient in food production - little known fact. Something about it not encouraging reliance on each other I guess.
Does the rest of the EU necessarily support each other in times of crisis - on paper they should, in practice they don't.
To sooth French sensibilities, an astounding amount of waste happens keeping centres of EU power in various countries - it isn't just in Brussels.
EU decisions are really taken behind closed doors between the French and Germans, then these are presented to the parliament. Don't believe me- ask your MEP, this one is a well know fact. Might have got diluted mind since the EU expanded.
They let in Romania and Bulgaria. What the hell were they thinking!
The EU has become ungainly, and can only survive, and must evolve by becoming a superstate, with common currency, laws, government, economic and social policy etc.
This might to some be a plus point. To others (Especially on the left of politics) it's a very sore point. The left wing of Labour have always opposed the EU as they have said it's undemocratic, because it's near impossible for the mandarins to be voted out. Tony Benn used to say this for years. Go figure there
Maggie Thatch campaigned on a pro-european platform in a very fetching dress of flags of nations. If there was ever a more turning off sight, then this was it.
So ok, a few of those were a bit tongue in cheek, but imagine if we pulled out, the uncertainty of such a move in the markets, and the impact on jobs. Would anyone seriously like to take such a risk :( I'm glad I don't have to decide such a thing