what is your experience? pros and cons ?
Since I am in the U.S., I unfortunately have
no experience with a 640G. Maybe sometime in 2016 … probably
late 2016 at that.

But it is a topic online which I am interested in. Here's what I think may be true based on what I've read so far.
Generally the feedback for the 640G has been positive. The things people tend to like are the color screen and the stronger support for it being "waterproof". I have using Medtronic/MiniMed pumps for not quite 20 years, so I'm so used to their user interface at this point that I don't think I could give a meaningful opinion. But others seem to think it's at least "not bad". Possibly even better in some ways than with other pumps? There is
finally a volume control for the audible alarms which I think folks have been waiting for since, well, forEVarrrrrrr!
The negatives I am aware of have not been anything which resonates with me so I'm not a good source of info for that. Apparently it may be slightly larger/bulkier than the Veo or (earlier) Paradigm Revel it replaces. Also, it still does not support a touch screen interface. You still navigate the menus by using arrow buttons. To some folks this stuff is really "a thing".

For me, not so much.
On its own, the 640G is a solid & nice enough pump. However, the feature of the 640G which in my opinion makes it unique at the moment is its support for predictive basal suspend and resume. When/if you can enable this feature and it works for you, it can be very helpful. Some people really like this feature and are impressed by it. It is certainly what interests me most about this pump.
However … predictive suspend & resume, which the Medtronic marketeers felt compelled to obscure by referring to it as SmartGuard™, can
only be used if you are also able to use Medtronic's CGM (Continuous Glucose Monitoring) sensor technology. There are two things to keep in mind about this.
The first is that it is/can be rather expensive. I have essentially no idea how funding for such things works or does not work in the UK. But the impression I have gotten is that many (most? all?) of the people who use it there are self-funding.
The second thing is to consider is that Medtronic's CGM is often considered to be "less special" by the #DOC (Diabetic Online Community). More precisely some people seem to absolutely loath Medtronic's CGM.
My point and opinion is that people's experiences can be extremely personal and vary widely. Some people have great luck with it seemingly from the first get go. Others have a hate/love relationship where the results vary from sensor to sensor. And of course there are also the haters.
If you ever decide to give this CGM a try in order to take advantage of the predictive basal suspend and resume and find that you must pay for it yourself, I strongly suggest you first get some sort of a trial with the CGM in order to find out how well it would work for you. If nothing else negotiate some agreement where you can get your money back if it turns out to not work out for you. Just sayin' …
Good luck with your pump and I hope you continue to get feedback about it from other folks. If you have questions, please feel free to ask. I personally would also be interested in how the process of getting you up and running with the 640G is done. I had a rather different experience when I started using the pump and I always like to hear what sort of process people currently go through when switching from MDI to a pump.