I recently purchased a new bike with the value of ~£500. After reading reviews of this lock i decided to invest in the money for peace of mind.
Size: The size of this lock is good; not to big and not to small. I was tempted in a smaller lock for portability but would have been limited to where I could have used it. it also seemed that many of the D-locks were quite big and long. I would place this lock in between. its big enough to wrap around most posts/gates but not to long to be a nuisance to transport around. This will just about clasp over my bike frame, back wheel and a smallish gate/post if I really tried.
Transport: this thing is heavy, but not ovary so. If you can put it in a backpack when riding its fine. It also comes with a mount which you can fix to your bike (this is what I do). The mount and clasp rotate so there are quite a few spots where you can mount it and its quite sturdy, often being difficult to get apart! I have also tried just locking it round my frame when I ride, but its a little big and heavy for this and gets in the way.
I would feel quite safe leaving my bike in town with this lock, let the nasty buggers go after easier pickings with small wire locks. I do however see many bikes with front wheels missing, so perhaps another smaller lock or wire in addition to this would be best to fend off these nasty buggers.
I hope this information helps :)