I have to disagree with fibernachi - there is old evidence that shields damage supply, but new evidence shows that there is no change in yield of milk through shields than without. This is likely due to modern shields being made of very thin silicone, with cut-outs so the baby's nose and chin touch your skin, whereas old shields were made of thick rubber (like the material hot water bottles are made from).
I fed for 5 months with shields, NO issues at all with supply and only stopped due to returning to work. When I began to use shields, i wanted to know why HVs and midwives were generally disapproving. I did a lot of research and found the studies I mention above, and asked on MN breast feeding board and found lots of women who had used shields for months and months without problems (like I went on to do). If you are interested, do a search with my name and you will find them.
A great website is a MNers blog called the truth about breastfeeding; she had the same issues, began to use shields and wanted to know what the problem was - and found studies to prove there's no issue these days.
To my mind (and from experience) if it helps you to use shields, do it, and for as long as you like. Medela shields were best for me btw.
Ok - essay finished!!! I really feel quite strongly about this after a really tough time with BFing prior to using shields, and get angry that their use is discouraged these days for no good reason.