Silicon Valley-spirited auto innovator Tesla Motors (NASDAQ: TSLA) has been hell on wheels. For those of us who were mulling investing in Tesla, well, I believe we blinked and we missed it.

Tesla shares motored over the $100 mark in no time flat. Although they've slowed a bit, right now the stock's been no clunker for people who invested in the stock somewhere between March and early May, not to mention last summer.

I have repeatedly considered adding Tesla shares to the Prosocial Portfolio I've been managing for Fool.com for more than two years now. I have also repeatedly chickened out (see my reasoning here and here.)

In some ways, Tesla would fit perfectly, given its innovative view on car production, its intensely green foundation (getting the world off oil), and darn it -- Tesla makes absolutely gorgeous cars. I'm not even a car buff by any stretch, and on the rare occasions that I've seen one in real life, I've had one response: an awed "oooooh".

However, Tesla's still got some devils in the details. There's its historic lack of profitability (granted, it recently reported its first quarterly profit), its current models' luxury price tags, and the build out of charging stations and anxieties about just how far Tesla's vehicles can go on one charge. The hardcore case of verbal fisticuffs between Tesla founder Elon Musk and a New York Times reporter over range issues also gave me cold feet. 

Built for speed, but what about the brakes?
Although things are looking good under Tesla's hood given recent news, it's hard to justify a 250% increase in price and a forward price-to-earnings ratio of 96. Some premium-priced stocks are OK in my book, but I don't believe Tesla has its future in the bag.

Amazon.com (NASDAQ: AMZN) is an example of a stock with an arguably justifiable premium. The shares always look expensive, but we also know about the massive appeal of Amazon's business and the breathtaking scope of its various product lines. It touches all of our lives one way or another. (Remember Kindle, Prime, the cloud, Zappo's, Audible, IMDb.com, and so forth.)

For investors who are looking for environmentally friendly automaker options, they may be surprised that Detroit isn't as old-school as one might think. General Motors (NYSE: GM) has a surprising number of sustainability initiatives, and many are admirable and impressive. One extremely impressive achievement: of 125 GM factories, 105 send no waste to landfills. Meanwhile, some investors consider GM a buy at these levels, and most probably aren't even factoring environmental responsibility into the equation. That may be a far smarter investment than what is now a momentum play that certainly might squeal to a halt.

Tesla's stock surge is certainly breathtaking in the short term, but I wouldn't buy at these levels. (I'm not the only Fool who feels that way, either.) If I had bought the shares at their lows, I might even consider selling now, although my core investing focus is long-term; if investors don't think they'll have the stomach for the very long road trip Tesla faces, they may want to get off this speeding ride now.

There's still a lot that's unproven about Tesla and its financial future, and the stock's swift ascent means investors could slam on the breaks very quickly, too. For those of us who didn't buy in early, that moment's exactly what we're waiting for.

Tesla's plan to disrupt the global auto business has yielded spectacular results. But giant competitors are already moving to disrupt Tesla. Will the company be able to fend them off? The Motley Fool answers this question and more in our most in-depth Tesla research available. Get instant access by clicking here now.

Alyce Lomax has no position in any stocks mentioned. The Motley Fool recommends Amazon.com, General Motors, and Tesla Motors. The Motley Fool owns shares of Amazon.com and Tesla Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.