I have been mulling over a reply for most of the day...
If you have a full(?)time job and run a veg/fruit/eggs/meat producing smallholding already I would think you just haven't got enough hours in the day to a)look after goats (milking, feeding etc) twice a day, and b) make good use of the milk produced.
A good quality goat gives about 6 ltrs per day at peak lactation (2ltrs go to each kid for about 4 months), even in midwinter you will still get about 3ltrs of milk per goat. To get a good allyear round milk supply it would be advisable to have two females and kid them alternate years. That means you have a lot of milk for about 10 months, and then one of them is dry for the last two months of her pregnany. A good goat can milk for more than two years, but it could be more difficult to get them back into kid if they have been empty for a long time.
So even in midwinter you will have to get "rid" of about 5 ltrs of milk per day from two goats. Making one lot of hard cheese takes most of the day, and you have to be around to check on it, do the next stage etc - I have found it difficult to do other jobs in parallel, but maybe I am too much of a scatterbrain.... Soft cheeses are easier, can be done overnight, then dried during the day, also easy is yoghurt (quite runny, but very tasty). In order to make butter from goats milk you will need a cream separator, they are about 100pounds from the Ukraine. Washing up all the bits and bobs for it takes quite a while. If your milk is about, say 4.5% butter fat and butter is about 80% fat then you get about 55g of butter per litre of milk. You will need a disposal strategy for the remaining skimmed milk... pigs come to mind.
I milk (at the moment) 3 goats (1 GG, and two BT) and have about 6.5ltrs of milk per day. We buzz the separator into action every three to four milkings, and I also make quite a bit of cheese (about one hardcheese from 4.5ltrs per week, also haloumi, garlic soft cheese, quark etc). I also feed a couple of weaners. I make butter and icecream from the cream (and being continental have cream in my decadent coffee...) - and yes we are self-sufficient in dairy. (except a wee pot of Yeo to kickstart the yoghurt every so often and some cheap cheddar to melt on maccaroneí cheese
)
However I spend about 45 minutes in the goatshed in the morning, same again in the evening, mine also get a lunchtime snack. As I am at home during the day I can let them out (and quickly back again in the rain) when I am around. I also go and cut branches for them from about late May until the leaves are off, usually once a day. Due to hay shortage last summer ours had freshly scythed grass (my OH's first job every other evening when he came home from his office job/1.5 hours commute) from about May until October.
You will need a relief milker - or a milking machine. (My OH doesn't do hand-milking and neither are my wrists anymore, so we went and bought a milking machine - 1500 pounds, but IMO money well spent).
There are quite a few start-up costs - goathouse, fencing, feeding equipment, goats (it does pay to buy BGS registered goats, so you can make an educated guess at their milk production capabilities and sell quality offspring). A livestock trailer is not essential, but being able to borrow one is. Goats come into season quite visibly/audibly and will need to be mated within the next 36hours, so flexibility in just being able to drive them to the (previously planned and informed) billy is also necessary.
Ongoing costs are: feed (hay, dairy nuts etc), vets (not often but if yes than it gets quickly expensive), annual CAE tests, vaccination every six months, worming and (in my case) annual de-lousing. Hay is a worry most years, our bales this year are 4 pounds for the normal small ones, and my 7 goats go through a bale every two days.
If they are not lactating they will be ok on mainly hay and only a handful of concentrates, if in milk you have to feed according to milk yield, in late pregnancy feeding up to make sure kid(s) is growing... so it pays to have them in milk as long as possible.
I am curently writing up all my costs and hope to be able to claclulate how much our milk is per litre - or maybe I don't want to do that...
Quality wise it's a no-brainer, we do not pasteurise our milk, and the ice cream alone is worth it! But my vegetable garden has suffered quite alot in the last two years (I also have sheep, hens, a couple of weaners most years, meat poultry on one kind or another... and two children that need to be driven to/from school).
So, food for thought for 2012 - but I wouldn't do without my goats anymore (I have started to show and now pedigree breed....they are very addictive!)