This series, brought to you by Yahoo! Finance, looks at which upgrades and downgrades make sense and which ones investors should act on. Today, our headlines feature a pair of new ratings buy ratings for electronic gizmo makers BlackBerry (NYSE:BB) and LG Display (NYSE:LPL). Before we get to those two, though, let's take a quick look at a company that covers the tech sphere itself -- IT analyst Gartner (NYSE:IT).

Gartner running out of gas
Calling Gartner "an open-ended growth story with strong execution, attractive financial characteristics, excellent competitive positioning and shareholder-friendly capital return" on today, you might think that Wells Fargo was gearing up to make a big endorsement of the stock. In fact, what Wells was really leading up to was a big "but."

You see, "open-ended" and "excellent" Gartner may be, but with its stock having run up 37% over the past 52 weeks -- nearly twice the gains on the S&P 500 -- Wells worries that the bull market in Gartner stock may have run its course. "The stock is currently at the upper end of its relative performance band vs. the S&P 500 and at the upper end of its EV/NTM EBITDA range on both an absolute and relative basis," warns the analyst. It also costs more than $70 -- well within Wells' fair value estimate for the stock. Accordingly, rather than endorse buying more Gartner stock, Wells' best advice for investors to day is to avoid Gartner until the stock's pricing becomes a bit more reasonable.

And I think that' good advice. Priced at 23 times free cash flow today, and an even more ebullient 37 times earnings, Gartner shares are richly valued for the 16% long-term profits growth that Wall Street expects it to produce. Granted, the stock could grow faster than it's predicted -- the average growth estimate for the IT services industry as a whole is more than 17%, after all, and Gartner is a better-than-average company. But with no dividend to protect on the downside and a valuation that would be almost as rich at 17% growth as at 16%, I see more risk than reward here.

Result: Wells Fargo is right to downgrade Gartner.

Prospects brighten for LG Display
Turning now to happier news, shareholders at Korean LCD screen maker LG Display awoke to brighter prospects this morning, as analysts at nearby Nomura Securities upgraded their stock to buy. This news echoes similarly favorable commentary out of Citigroup last week, which says the stock is at a "trough valuation" of just 70% of its likely book value for this year, and is likely to outperform the market.

Citi thinks LG Display could easily earn $1 a share this year -- 25% more than the consensus estimate for the stock. If it's right, then the shares are selling for only 12 times near-term earnings -- in an industry where most analysts are expecting 19% long-term profits growth and in which analysts cited on S&P Capital IQ expect LG Display to post 40% or better profits growth.

That certainly suggests that Citi's and Nomura's enthusiasm for the stock is well-founded. Really, the only thing that worries me about LG Display is the fact that, while its GAAP profits look good enough, actual free cash flow at the firm remains rather week. Over the past 12 months, LG Display reported "earning" more than $400 million. But real free cash flow at the firm was a mere $106 million.

Even if you grow that number 40% year after year for five years, it only gives you $570 million in free cash flow five years from now. This means that right now, today, LG Display shares sell for 15 times the free cash flow it might possibly earn five years from now. Suffice it to say, this seems a large price to for profits that may or may not materialize far in the distant future. While I like LG Display as a company and respect its products, the stock still looks overpriced.

Picking on BlackBerry -- again
Last and least, we come to Canadian smartphone maker BlackBerry, which scored an upgrade from CLSA last week (as you may recall) and is on the buy list again today -- this time winning plaudits from the Toronto analysts at Cormark Securities.

Cormark notes that "time is ticking on driving a sustainable business" for BlackBerry, and in more ways than one. BlackBerry is due to report Q4 earnings this Friday, and Cormark believes these earnings will show "a continued decline in the core business," with revenues of less than $1 billion missing estimates by 18%, and losses being far greater than many investors are prepared for -- perhaps as bad as $0.78 per share.

But if all that's true, then why would Cormark recommend buying the stock? Simply put, the analyst is betting that "newsflow and non-operating changes... will continue to drive the positive sentiment" on this stock that's already gained 23% so far this year. Put another way, Cormark is arguing that despite BlackBerry's business continuing to deteriorate (I mentioned last week that the company has no profits, nor any prospect of earning profits as far out as analysts can see), you should buy the stock anyway -- in hopes that other folks will also buy it, keeping the price going up.

Such a strategy might make sense so long as the world's supply of "greater fools" doesn't run out. But once it does, look out below.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.