I love the idea of working out with a personal trainer, but as a working mom on a tight budget, it’s not really an option either time- or money-wise. That’s why I’m loving this trend of “virtual” trainers, where you download personalized programs designed by experienced trainers to help you achieve your fitness goal, then do the workouts on your own time—all for a monthly price equivalent to that of a single session or two with a live trainer. Time to test the newest entry in this crowded field, the NextFit Keychain trainer.

The Keychain is about the size and shape of a cigarette lighter, and it could indeed go on your keychain, if you were so inclined. (I’m not.) To keep it close by during your workout, it comes with both a lanyard and an iPod-style armband. Take my advice and go for the arm band—hanging from the lanyard, the Keychain bounced annoyingly against my chest while doing the moves. But I’m getting ahead of myself.

When I first plugged the Keychain into my hard drive and downloaded the program, I was taken directly to the sign-up page. Then, I was asked a series of questions: my goal (“Shape and Tone”), BMI (body mass index, which they helpfully calculate for you when you put in your height and weight), fitness level, workout experience, and preferred activities, plus where I work out most, what equipment I own, and how long I want my workout to last. All good questions. After registering my answers, they gave me three program recommendations, each from a different “star” trainer. I chose the first option, a weight training/body shaping program with Kathy Smith, star of a gazillion workout DVDs.

Then, I clicked to download my first workout. And waited. And waited. It took about 10 minutes to load, which seemed like a lifetime on my usually speedy computer. (But hey, at least I didn’t have to take the time to drive to the gym to meet with my trainer.) Downloading subsequent workouts took just as long, but were admittedly to do—I just plugged the Keychain into my hard drive, clicked on the NextFit logo, and was taken directly to my personal workout page where my next workout was waiting for me.

One of the cool things about NextFit is that they don’t just pair you with a trainer; they give you a choice. After three workouts with Ms. Smith, I was ready for a change—the exercises weren’t challenging or interesting enough, and she seemed to vacillate inexplicably from cheerleader to let’s-get-it-over-with mode. So I switched over to celeb trainer Ron Mathews, who turned out to be much more my speed and style.

Another cool thing: Each time I came back to load subsequent workouts, I had the option of clicking on a link called “Improve your workout”. How could I resist? I was brought to a page where I was asked for my thoughts on the previous workout’s moves and intensity, plus whether I wanted full instructions before every move, how much motivational chatter I wanted to hear, even whether I wanted to hear my name or not during the session (no thanks—too contrived). I found it to be a really helpful way to make my workouts more like I wanted them to be—in other words, some different moves, more intensity, fewer instructions and rah-rah talk.

I only found two things not to like about the NextFit Keychain Trainer, and they’re both minor. One is the earbuds. I’ll admit that my ears are a bit on the small side, but earbuds usually at least fit into them. These did not, despite persistent cramming. So I finally gave up and switched over to the ones I use with my iPod—problem solved. The other thing is that the control buttons are a little hard to read. That, too, became a non-issue after I’d used them a few times and knew what was where.

Usually when I test this kind of product, I leave it behind once I’ve written my review. Not so this time around—I’m fully planning to continue following Mr. Mathews’ workouts through the rest of the month. And who knows? I might even renew.

Product: NextFit Keychain Trainer

Category: Gear

Pros: A highly personalized workout program with a wide range of trainer options and lots of opportunities to customize your routines so they’re exactly what you want.

Cons: Slow to download, plus the earbuds are too big to fit some ears. Also, the buttons are a little hard to read.

Cost: $99.95 (including first month’s subscription; $24.99 for subsequent months) at www.MyNextFit.com/AlyceaUngaro

Extra tip: Don’t like the music? Click on the easy-to-miss “Experimental Features” link on the bottom of the main page when you log on, then add your own mix from your iTunes library.