We stayed 10 nights in India Guesthouse (we had to stay that long in Mumbai and with our budget it was the only hotel we could afford). The room we got was very nice, quite big, with window, a desk, double bed and there´s free wifi (that sometimes works, though). Rs$600 per night. The toilets are shared but they are kept very clean and with toilet paper, so thumbs up for that. The staff is very helpful too.
The location is the best in Colaba, so that´s something to consider.
Bad things: the walls don´t reach the ceiling, so when you are sleeping is very easy to wake up with your neighbor´s light, people talking, music, etc... And because of that, there´s also not much privacy... even one day a girl that was staying next to us climbed the wall and said "hi" from the top of it... very weird.
Our room was next to the room where the staff sleeps, so EVERY morning at 6:50am we woke up with their alarm clock and all the talking that came after... So, after a couple of days that was VERY annoying.
In conclusion, best price in Colaba because Mumbai is so expensive, clean rooms, great location, but consider it just for a couple of nights.