Since I wrote the piece below various things have changed (for the better).
First, some of the various party views have changed, mainly, it seems, as some of the details have become better known, especially the secret courts by which corporations could sue governments over policies which hit their profits.
Second, some European governments (notably but not only France) have declared their opposition to the proposals, so there is a good chance it will all be cancelled.
Third, it has become clear that if we leave the EU our present UK government are quite likely to conclude a similar deal with USA, so leaving the EU would not solve the problem and might well make it worse.
This treaty is being negotiated in secret between the EU and USA and amounts to a complete corporate takeover and an end to effective democracy. This link is the best explanation of the problems with it that I have found so far:
I note that 38 Degrees have also started a campaign against it. See: https://secure.38degrees.org.uk/ttip-leaflet-action.
Of the UK political parties, this promised disaster is supported by Labour, Tories, LibDems and UKIP, and opposed only by the Green Party and the Scottish and Welsh nationalists. Mussolini would have loved it.
While I would be reluctant to see us leave the EU (although it needs a lot of reform), if this goes ahead I would regard it as sufficient reason in itself to do so.