I've stayed at Bally's twice now, once in newer rooms and once in the cheapest room. The newer rooms are comfortable, on par with any mid-scale nice hotel chain. The older rooms are...old and worn. But if you need to stay on a budget and want to be in the middle of the Strip, there are few better choices. It's true, the location is really hard to beat. This is especially true when using the monorail since there's a station attached. "Who uses the monorail in Vegas?!" I do - it works.
I also like the table games at Bally's. They're usually cheaper and don't have hardcore players at them. I found a $5 Texas Hold'em table that was a blast, especially for those who aren't serious poker players but want to play (the tournament rooms on the other hand are too intense). There's also sports betting and cheap food options in-house.
So not the nicest place to stay, but if you're on a budget or can go with an upgraded room, Bally's is just fine. Plus you're in Vegas, you shouldn't be in your room much anyway, right?