Would you recommend using the discount airlines in Europe? If so, why?
If not, Why not?

Dear Tester, 

Having used these airlines, I would recommend them. There are several reasons: 
(1) The price: when booked well in advance, they are *really* cheap;
that means that travelling is much more affordable.
(2) What you get for that price (or, what you don't get): you don't
get food and a movie. Who needs it anyways? You don't lose in
timeliness, security, and the really important things - they are the
same as with a "regular" airliner.

The disadvantages (when I wouldn't have taken such a flight): 
(1) They sometimes land in remote airports, or not the main airport of
the city. For example, they land at London-Stansted, not in Hethrow.
If that would ever be a problem (e.g., I have to catch a connecting
flight and would prefer the same airline, or at least the same
airport), I wouldn't use the budget carriers.
(2) If I want to slow down and see the country. The flights are
naturally functional. If I will be travelling, I might want to get to
see the way between the places.

In addition, extreme ecologically minded friends of mine have told me
that these short flights consume lots of petrol, therefore posing some
environmental hazard.

I would still vote "for" these cheap flights, of the reasons mentioned
above. I recommend this site to check those airlines:
WhichBudget
http://www.whichbudget.com 

I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it.

I live in the UK, and use the main UK budget airlines - EasyJet and
Ryanair - on average 5 or 6 times a year. My experience is that they
are reliable, have an excellent safety record and keep very good time.
On the other hand, as they themselves state, they are "no frills", and
you don't get seat allocation, TV or free food (you can buy snacks &
drinks) - but this doesn't bother most people because flights within
Europe are generally fairly short. The great advantage over the
scheduled airlines is of course cost, with many budget flights costing
only a quarter or less of scheduled fares; and the cheapies
(especially Ryanair) fly to far more European destinations than any
single scheduled airline. I've also used some other countries' budget
airlines - especially Germany's Air Berlin - and have good experiences
with them.

I'll second both postings, but mention another disadavantage:  The
fantastic prices may be for a limited number of seats, and if you want
to change a reservation it can cost you significantly.  You should
also remember to include the not insignificant cost of getting to and
from a more remote airport when comparing prices.