I've been to Udupi Bhavan and recommend it highly. We were there on a Saturday for lunch. The place was fairly busy. We ordered a starter of idlis, DH had the thali which he raved about. I had a surprise masala dosa, something the chef made which wasn't on the menu. It was terrific, very tasty with a filling of various vegetables. It's on our list of favorites in the area.
Here is my previous post on this place, sorry for the recycle, in a bit of a hurry.
Also in Lowell is Mysore Veggie. Pretty similar to Udupi Bhavan, maybe a little more ambitious, with some chinese/indian fusion dishes. Also excellent falafel which was a nice surprise. In the plaza with Family Dollar and the Post Office right before Udupi Bhavan.
1689 Middlesex St.
Lowell, MA 01851