COI: I peddle epidurals for a living but haven't actually had one myself (yet).
Epidurals have risks (for full info please see the information that the Obstetric Anaesthetists Association publish in their information for mothers section) www.oaa-anaes.ac.uk/
No one, least of all us anaesthetists would try to tell you otherwise. Only you can weigh up the risks and make a decision for YOU...however the following may help.
Modern labour epidurals use much lower concentrations of local anaesthetic than have been used in the past. In the unit I work in most woman who have a 'standard' epidural will be able to move around and change their position in bed.
We also offer 'mobile' epidurals which give smaller doses of the same mixture. In general, these are less efficacious in terms of pain relief but you can mobilise. We have 'telemetry' CTG monitors to allow woman to mobilise freely with a portable monitor pack. I generally advise people that if they are motivated to get up and move, stand, bounce on a birthing ball then a 'mobile' is a good choice. If they dont' get adequate pain relief extra drugs can be given to make it into a normal epidural.
I 'm not sure why people in our unit are so reluctant to mobilise with 'mobile' epidurals. They do all have the physical capability to do so - maybe they need to sleep once they get adequate pain relief, maybe the midwives aren't supportive. Some woman definitely do mobilise though.
With regards to pushing - research shows that women who have epidurals have an increased risk of instrumental deliveries - the mechanism of this is unknown. Your epidural should not be allowed to wear off completely for pushing as suddenly hitting you with lots of pain when your body hasn't been allowed to acclimitise would be very very sore. However the midwives often try to time it so that you don't end up pushing when your epidural is at it's most effective.
My experience suggests that a lot of the time people end up with epidurals because their babies are malpositioned and consequently labour is more arduous than you would otherwise suspect.
My own birth plan: I'd have an epidural if I needed one but would prefer to 'see' if it is necessary by exploring other options first.
If you are having one you do need to accept the biggest risk which is that it may not work perfectly (see gigglewitch's post above regarding a unilateral/ one sided block).
For certain women, who are at high-risk of complications, it may be beneficial to think about having an epidural early as once one is in and working it can generally be 'topped' up to provide a rapid block for operative proceedures. Having one in can avoid the need for a general anaesthetic in an emergency situation (normally we would just put in a spinal anaesthetic) which is thought to be more risky for you, your baby and from reading previous mumsnet threads appears to be considered to be a generally bad thing!
Sorry this is a bit of an essay