Call them irrational, but Sirius Satellite Radio (SIRI) - Get Report stock maniacs continue to find a logic of their own.

Sirius plunged 18% Wednesday after getting hit with a pair of downgrades. The retreat, which came a day after the stock hit a multiyear high, shows how analysts who just a few weeks ago cheered the stock now are finding it harder to justify its expanding valuation.

But for some, Wednesday's pullback is a wish granted, offering a long-awaited opportunity to get into the stock more cheaply.

Sure, even some longtime bulls admit Sirius' $9.01 close Tuesday was arguably nutty. But even nuttier perhaps is the idea that buying at $7 helps make up for the mistake of missing the recent run from $4 to $9.

In other words, the cardinal sin in growth-stock investing isn't picking the wrong stock -- it's missing a chance to get in.

"This is an opportunity to get involved," says one New York hedge fund manager who had previously scoffed at the price yet anguished over not catching the rise weeks ago. The money manager says he bought Sirius on Wednesday on the selloff, and says he's looking to play the stock for quick trades as opposed to building a purely long-term position.

"Is the valuation ridiculous? Of course it is, but so was RIM a year ago," he says, referring to

Research in Motion

(RIMM)

, the maker of the Blackberry mobile email devices, and a stock that has quadrupled this year.

RIM's pager, with the miniature keyboard, gained an ardent following among a relatively small segment of the population. Its core users were mostly info junkies like analysts and traders, who could ill-afford to be out of reach of their emails. Bears called it a cultish niche product, and of course bulls said the sky was the limit.

The RIM story not only provides an interesting parallel to the satellite radio proposition, but it is also an example of how buyers build a rationale for their decisions. For example, when telecom-parts maker

JDS Uniphase

(JDSU)

shot up more than 1,000% in 1999, it built a bullish case for all companies with optical-networking aspirations.

So what makes Sirius any different than the soar-and-burn stocks of the past? This is where the logic gets interesting.

Fans say Sirius, unlike other companies in tech, has only one true competitor,

XM Satellite Radio

(XMSR)

. And as Bear Stearns analyst Rob Peck argues, this isn't VHS vs. Betamax -- it's more like Coke and Pepsi. The satellite radio market will be divided between Nos. 1 and 2.

So how big is the market? Sirius Chairman Joe Clayton sees 350 million potential users. And while that figure is considerably greater than the U.S. population, fans say it's not outlandish if you allow that people will enjoy the service enough to buy multiple radios for their cars, boats and homes.

Then there's portability. A new crop of handheld satellite radios are coming to the market, and the bulls see a popularity that could rival

Apple's

(AAPL) - Get Report

iPods.

But at $9 a share, Sirius is valued far more richly than profitable companies like

AT&T

(T) - Get Report

and

ConEdison

(ED) - Get Report

and

Maytag

(MYG)

. Perhaps even more alarming, at $9, Sirius was trading at twice the valuation of XM, which at last count had three times as many customers.

A former bull turned bear says Sirius simply doesn't have the catalysts ahead in terms of subscriber numbers or programming deals, like another Howard Stern, needed to justify even its current $7.50 price.

The New England-based money manager who sold his Sirius position last month says the stock won't hit $9 again for two years or more, and that "the last $3 were just plain stupid."

But using the topsy-turvy logic, Sirius had recovered a little lost ground Wednesday. After being down as much as 27% earlier, the stock recovered a bit to trade at $7.48, just 17% down.

That's probably a win in the Sirius logic books.