Specifically, a level 1-49 imported Shepard starts ME2 at level 2, a level 50-59 Shepard starts al level 3, and a level 60 Shepard starts at level 5. While starting ME2 at level 5 is a good thing, I wouldn't put up with an ME1 NG+ game to do it. ME1 combat starts to fall apart after level 45 or so, and loot will become worthless after about level 55.
Note that the ME1 "Rich" achievement gives a cash boost in ME2, so managing your item buying in order to pass a million credits is useful.
I just don't like starting an ME1 insanity run with the crap weapons you have at the start of the game and a Level 1 Shepard who can't hold a sniper rifle still enough to hit the broad side of a barn door... and I wanted the insanity achievement. So, since I was doing the ME1 insanity run anyways, it made more sense to me to just complete it and make it enough of a complete game that I could import it into ME2 (as opposed to just doing a bare minimum insanity run and ending up with a saving I wouldn't want to import). I did my first ME1 game romancing Ashely, renegade and the second Level 60 game romancing Liara, paragon. That way I had two different fairly complete games I could import into ME2 and then into ME3... and get a pretty full experience from both (i.e. all the emails, all the old characters that come back, etc.). Now, I tend to do more minimal playthroughs in all games, just focusing on those characters I really want and not concerning myself with, for example, saving everyone in ME2. I also do a lot more blending of specific paragon and renegade sections (so my character overall doesn't lean heavily either way) and I get to hear some different lines each time.
To ensure I get the Rich achievement - Near the end of the game, I usually end up just completing Dr. Michel's assignment (so she pays me more for selling her guns) and then I do some "gun running" (buying my own guns back from another merchant for less than Dr. Michel paid me for them).