@JTBmum - you can get 2 Day seats per person I think...my partner went a lined up early for us to get them a few weeks ago. To be honest, we didn't have the best seats. Usually with day seats you get good seats near the front, but sadly for this they were right at the back of the stalls.
The front row (which are day seats) are also a little bit too close to see everything. Choose these over the back of the stalls though (IMO) as you will have a much better time.
Maybe it is worth spending a little more (day seats were still 25 pounds each!) and buying good seats. There are some deals around as it isn't selling as well now the initial rush has died down.
Reviews have been really positive on the whole, although the odd newspaper picked holes. Their website actually features reviews from blogs which is a new touch, but is interesting that they seem to like it aswell.
http://www.sweeneytodd.co.uk/reviews/
Has anyone else seen a Sondheim show before?
@katmandu - thanks for your tip on the Royal Circle. I would probably rather pay 10pound more and have a good seat rather than at the back if I do go again (and I really hope to!) Comedian Jason Manford is joining the cast very soon so he will be worth seeing it again for!