Last week, I introduced five stocks for the greedy investor's watchlist. In it, I promised to reveal which of the five I'll be including in my Roth IRA. I'm here today to tell you that Zipcar (Nasdaq: ZIP) will be earning a seat at my Roth IRA's table.

Why I'm buying
I'm in the middle of reading Clayton Christensen's brilliant book, The Innovator's Dilemma. In it, Christensen talks about how several new, young companies are able to usurp their much larger and more mature competitors by introducing disruptive technologies.

While Zipcar, which provides a car-sharing program for the carless in both large urban areas and college campuses, hasn't really introduced any disruptive technology to the automotive arena, I think it's actually on to something even bigger: a disruptive idea.

In creating a subscriber-based company that offers its Zipsters all the benefits of car ownership -- namely, timely transportation and the ability to move large objects -- with none of the hassles -- like paying for gas, looking for parking spots in crowded downtown neighborhoods, or worrying about insurance -- Zipcar is looking to disrupt two industries.

An angry Detroit?
Typical titans of the car industry like Ford (NYSE: F) and General Motors (NYSE: GM), even though Zipcar buys and leases from them, stand to suffer from slower sales as customers opt for the convenience and value proposition Zipcar offers over the hassles of ownership.

Even Tesla (Nasdaq: TSLA) could see its attempts at profitability thwarted. As wealthier but environmentally conscious consumers consider their options, they may choose car-sharing over electric vehicles as a better option for the environment.

Skipping the rental counter
On the other side of the equation, we have car rental companies like Avis (Nasdaq: CAR) and Hertz (NYSE: HTZ) that could see their rentals being sliced away as well. If I land at D.C.'s Reagan National Airport, why would I rent a car when I can take the Metro to a Zipcar and use the car at my behest for far less?

It should be said that Hertz isn't going to take this lying down. It has offered a similar service to Zipcar's. Hertz On Demand offers hourly rentals with insurance and some gas covered, and it does this without a membership fee.

But Hertz has two distinct disadvantages. First, its on-demand offering is in far fewer cities and campuses than Zipcar's. Second, when I take a quick look at where Hertz's rentals are in the D.C. area, there are only two locations -- both near Union Station. Although this location makes sense for visitors, Zipcar has tons of members (600,000 and counting) who use the service for daily errands in their home city. For them, this one location would be wholly inadequate.

One last disruptive characteristic
One of the keys that Christensen points to in disruptive technologies (or in this case, "ideas") is that at first, the product is only adopted by a small niche market that slowly grows to envelop much more than initially projected.

With its initial focus on four distinct urban areas and college campuses, the same can easily be said for Zipcar; it has started with a niche market. Moving forward, it will be difficult to analyze its market potential (both in the U.S. and Europe) because much of it depends on consumers fundamentally changing their relationship to automobiles. Only time will tell how large the market opportunity really is.

Get started today
There are two reasons I'm starting this series. The first is because I'd like to establish a public record for how my stock picking is doing in a forum that will help hold me accountable. By announcing one stock a month that I'll be investing my Roth IRA money in, I gain that accountability.

But the second reason is that I think Roth IRAs are underutilized by many of today's newer entrants to the workforce. Though things like building up an emergency fund and paying off high-interest debt need to come first, contributing regularly to a Roth is one of the most important steps you can take in your investing life.

I hope that my public nods to Roth IRAs will encourage others to take advantage of their own accounts.

An IPO? Really?
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