The commute out on I-90 (Mass. Pike) would be a reverse commute, so that part should be no problem. Once on 95 (known locally as rt. 128), you could encounter some serious traffic. I'm not really familiar with the back streets, but as I picture the area between Newton and Lexington, I'm thinking your best commute would probably be via local streets, though it might depend on what part of Newton you lived in and where in Lexington you would be working.
Lexington does not have any kind of rail service on the T, but it is served by transit busses. Newton is as well. I'm not very familiar with bus service, but it would be worth a look at the T's website (
http://mbta.com/schedules_and_maps/bus/) to see whether there is a route between Newton and Lexington.
These towns are really pretty close to each other. There is only one town between them. Newton has a lot of good qualities. If I really wanted to live in Newton to take advantage of those qualities, I would consider the commute to Lexington to be quite reasonable for the chance to live in I town I really liked.