I'd wanted a homebirth first time around but unfortunately ds1 had other ideas as my waters broke at 36+2 so I wasn't allowed a home birth. As our local labour unit (the only other option in our area) was being refurbished they'd stopped all hospital visits so I had no idea what it was like there. I won't go into details but the labour was ok although there was lots of intervention, the aftercare was awful!
So, second time around I was determined to have a home birth. low & behold I went 4 days overdue that time :-). I had an absolutely wonderful experience and can't recommend it enough. It was much, much quicker at home, which could've been down to being more relaxed in my familiar environment and being able to do/watch/listen to/eat what I liked when I liked. I sat on my birth ball most of the time, sipped lucozade and sucked on gas & air. The main thing to keep in mind is that should anything go wrong or you're not coping well etc, you can always transfer to hospital.
The first thing to do regarding a homebirth is to let your midwife know that you want one. She can tell you the procedure in your area and talk you through things you'll need etc. If you come up against any resistance there are places you can turn too, I'm not sure where but if you do an internet search you will find the authority to speak with. I'm lucky as we live in an area where the m/ws are very pro-home birth so have not had to deal with this problem. Also, once you've gone into labour and contacted the m/w make sure you or your partner inform the m/w or receptionist that you're planning a home birth and that you want gas & air (if you wish to use it), that way the m/w should turn up prepared to stay rather than just to examine you and arrange hospital admittance. For a list of all the things you'll need for a home birth there's a great thread on here somewhere with loads of info/ideas, I found it really useful even though I've done it before.
A home birth is no more or less safe than a hospital birth. Your m/w will carry everythign with her for all eventualities and should also set up a resus area just in case there are complications after delivery. It really is just a precaution but a very wise one. m/w will regularly monitor baby's h/b with a doppler to make sure he/she's happy in there. Of course should any problems arise during labour you can be transferred to hosp immediately. Evidence shows that in general women who labour/deliver at home are less likely to need intervention of any kind (ie, assisted delivery), even if they are transferred to hosp.
With regards to reasons not to have a home birth it would depend on your personal situation. Such as a breech baby, low iron etc. This is why it's essential to let your m/w know asap that you'd like a home birth as they can monitor these things in the later weeks and if needs be turn the baby so you can still have a home birth, give you iron supplements etc.
I'm on baby No3 now, our last. Due any day now so I'm in 'nesting' mode and almost everything is ready & I can't wait! I've brought a birthing pool this time too & am very excited. Just got to keep my fingers crossed now that there will be a free m/w available when I need one :-)
Wishing you lots of luck and hope you get the birth you want!