I started an internship a month ago at a biotech/pharma. I'm having a great time and learning a bunch about industry (this is my first time working in an office). I'll still have about six weeks to travel after the internship.
The decision about whether to do a pre-MBA internship is a personal one. A lot of people will tell you to take time off to travel and relax, and I wholeheartedly support that. But as I said, I've never worked in an office before. I wanted some experience with numbers and transactions prior to re-entering the classroom. And if I end up gunning for a VC or start-up internship next summer, I'll have something non-medical to put on the resume. The only other person I know who's doing a pre-MBA internship is in a similar situation: just finished a long military stint, spending the summer with a consulting firm.
One piece of advice about looking for pre-MBA internships- leverage your connections. Placing the name of your business school at the top of the education section of your resume is fine and well, but you don't have the advantage of career services and on-campus recruiting/networking. I'd estimate I sent out about 8 or 9 unsolicited CVs/resumes (via web-sites, etc). Didn't get a single call from any of those firms... heck, I didn't even get an email. I was able to send another 5 CVs/resumes to equivalent firms via contacts (i.e., my connection contacts someone in the firm, who emails me asking to send the CV directly to them). Got interviews with 3 of those 5 firms, with offers from all 3.