I have been on 1mb for quite a while too and can recommend it if your area isn't oversubscribed. My area is and it can be hell.
During off-peak hours (usually) download speeds are great. Browsing web sites with lots of pictures/movies is a joy. But when everyone else gets home from school/work, or during holidays (today, Bank Holiday Monday, for example)it's worse than being on 56k dial-up!
You haven't mentioned your reasons for wanting to 'upgrade', but if you're a gamer, be aware you won't notice any reduction in your pings. So do it for downloads and rapid browsing of content rich websites, but be prepared for slow-downs if your area gets busy.
Thanks for the responses I noticed that you can revert after two months so I have given it a go so far downloads much faster as for general browsing haven't noticed that much difference but then its early days.
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