Depends on what your training for mate, if its to improve your running and fitness i would suggest that you need to get out and put in those miles either on the road or cross country to get the full benefit, obviously a softer surface such as grass or sand is better than tarmac on your joints but you have to work with what you have at the time so a compromise is usually involved when your training regularly...
yeh it depends on what you want to use it for, i personally get bored on them if on for any longer than 20 mins. any longer than that & i get out side.
They are useful for pacing etc as you can see exactly how fast & how far your going, So working on Speed is made easier than outside. Also if your lucky enough to live in an area of the country that is flat- you can do some hill work on them!
They can also be good at looking at running economy & style.
but ultimatly its best to train specific. so get outside.
treadmills are brill for improving your running times. Yes i know its not the same as running outside but think, as long as your running n breathing out your arrse hole then your working hard true? With a treadmill you can keep the same speed over a certain time/distance so you can measure your performace and improvement better Imho treadmills are amazing things if you use them right.
hate them hate them hate them cant get a goo0d candence feet slap like mad always some fat bint wobbeling her arse on a 15% incline on the one in front ewwwwwwwwwwwww i find the eliptical much better and for full on cardio torure the rowing machine tops my list 45 mins intervals on that at 1.1 ratio (1 min all out fast as you can ie level 8 1.50/250m at 32spm then 1 min recovery at 2.30/250m32spm) all but kills you but the gains you make are fantatic low impact so good for joints and builds all over cardio fitness and stamina plus unless your very unlucky you dont see the wobblers mince on the running machine