I have one, had the predecessor for 2 years and then this after I smashed mine in August so if you have any questions let me know!
I don't use either the Garmin Training Center Software or Motion Based. I've used both and I will occasionally load up to GTC, but MB was just more time and I log my running at
www.runningahead.com and didn't need ANOTHER place to log.
Sass - I do think if you are creating custom workouts the GTC software works quite well. You can do what you want several different ways.
If you are doing even intervals (90 seconds walk/90 seconds run) you can do Training/Alerts/Time&Distance/ change your time to 90 seconds. This however means EVERY run it will beep EVERY 90 seconds until you stop it.
You can also go under Training/Workouts/Intervals/Time&RestTime if your walk and run intervals are CONSISTANT through out the workout (separate from warm/cool) like 90 sec run, 2 min walk and set up a workout there on the fly. Include a warmup (check box) Start it FROM THAT MENU and then when you are ready for your first run interval hit lap and it will beep times at you from then on with a 5 second warning . It will save your settings until you change them (except for the warmup part) but you have to start it from the training/interval/time&rest time menu each time.
The best bet if you have a program that is more complicatied something like "Walk 5 min to warm up, run 90 seconds, walk 2 min, run 120 walk 3 min etc. etc. etc. " is to build a custom workout. You can do it in the Garmin itself but it is a PITA (Training/Workouts/Advanced), or you can build the workout in the GTC software - once you play around with it its not that hard. Then you upload it to your watch and away you go! This is a good way if you are following something like C25K because you can set up 3 or 4 workouts to match whatever the week is, and then each week just EDIT them and reload them.