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Zipcar shares made a peppy debut Thursday. The Cambridge, Mass., car-sharing service raised $174 million in an initial stock offering, with deal participants paying $18 for shares, well above the $14 to $16 the company had expected. Once shares became available to the public in open trading, the price promptly jumped to near $30. That values the company at $1.2 billion. Its sales last year were $186 million.

Is it too late to get in? Here's a look at some pros and cons.

Pro: Speedy growth

Since 2005, Zipcar's customer base and sales have each multiplied more than 10-fold. Last year, sales rose 42%. The dominant theme among U.S. companies over the past two years has been minimal sales increases offset by cost cuts, which makes Zipcar a rare source of genuine fast growth.

Con: No profits (Yet)

Zipcar has operated for more than a decade without earning a dime. That's not as worrisome as it might seem, however. Last year the company lost a modest $14 million. Its operations are profitable in key cities like New York and San Francisco. Young companies often turn in losses while spending all they take in and then some to expand. The fast sales growth suggests profits will appear soon, especially with the debt reduction that the stock offering proceeds will provide. Management says it doesn't expect a profit in 2011, but it might just be tempering expectations.

Pro: Big market opportunity

The company says it has an overall sales opportunity of $10 billion worldwide. Last year it bought a small London car-sharing service called Streetcar.

Con: Best U.S. markets might already be exploited

Cities where Zipcar is thriving, like New York, Boston, San Francisco and Washington, D.C., are unique, not just because of their large populations, but because of their walkability. Car-sharing services thrive where people walk--even people with money--because elsewhere, people with money own cars.

Pro: First-mover advantage

Zipcar is the biggest company in the car-sharing business. The company says it has cars within walking distance of 10 million drivers.

Con: Success lures competition

Hertz Global Holdings (HTZ) and Avis Budget Group (CAR) together have yearly sales of 65 times that of Zipcar. In December 2008, Hertz launched Connect by Hertz in New York, London and Paris and has since expanded into Berlin and Madrid. Cars are distributed through parking garages rather than rental offices and pricing is "in-line" with Zipcar, according to a spokeswoman. "We're watching our competitors closely and we're growing in a disciplined manner," she says.

Pro: Momentum

Simplistic as it sounds, stocks that are rising are statistically more likely than not to continue rising over the next few months (but to decline thereafter), studies show.

Con: It's an IPO

For every stock offering, there's an informed set of investors who are willing to sell part of their stake to the public. Long-term studies show that shares tend to underperform following stock offerings.

Pro: It's popular

Zipcar is not only a hip, recognizable consumer brand; it's also a "green" investment of sorts. Management says that each of its cars keeps dozens of other cars off the road by eliminating the need to own. Other green-themed stocks, including wind and solar companies, have attracted plenty of investors in recent years.

Con: It's popular

How expensive is $1.2 billion for Zipcar? It's 6.5 times trailing sales. That makes it more than four times as expensive as the average company in the S&P SmallCap 600 index--and more than 80% of those companies are profitable.

Correction: A previous version of this story incorrectly reported that Hertz had not yet entered the car-sharing business.