First of all, when it's July or August and there's a heat index of 115, it's just impossible to run as fast as you need to without risking heat exhaustion. So, a speed workout on the treadmill is definitely better than no speed work at all. There is a little bit of controversy as to whether or not there is much difference between speedwork on a treadmill vs. speed work on a track. Seems that most believe the biggest difference is the lack of wind resistance on a treadmill. To compensate for this you need to adjust the incline of your treadmill. The amount of incline depends on your pace. For example, if the treadmill MPH is set at 7.5 and the incline is set at 0%, the pace on the treadmill will read 8:00 min/mile. However, if you were outside you'd probably be running around an 8:20 min/mile. To account for the wind resistance, set the treadmill at a 1% incline and then your pace is more equivalent to a 7:59 min/mile out on a track. Hillrunner.com has a great chart that calculates all the equivalent paces by incline for you for from 0% all the way to a 10% incline.
A smaller camp doesn't think wind resistance really is a factor unless you're running faster than a 6:30 min/mile pace. Personally, on a really good day, I might be running a 6:30 min/mile pace, but even when I'm running at a 7:00, 7:30 or even an 8:00 pace on a treadmill at a 0% incline, it doesn't feel the same as running outside. I don't know exactly if it's due to wind resistance or just the often unnoticeable roll of the landscape, but I think running outside is not truly replicated on a treadmill at 0% incline. If you're just trying to get in 30-45 minutes of running and you're not worried about pace, then keeping the incline at 0% is perfectly fine. If you want more of a true "outside run" or you want your speed work to be more akin to your running on the track, I'd suggest upping the incline by at least 1%.
Some treadmill companies have gotten savvy to runners wanting to do intervals on treadmills and have added intervals to their program options. Some LifeFitness models have a program set up for intervals. You enter your jog speed and run speed. Then toggle between the two with the hit of a button. To me this is a little more akin to fartleks, but check it out, give it a try, and see what you think.