The PlayBook tablet computer, Research In Motion's pocket-sized challenger to the iPad, feels rushed into service and limited by its reliance on a BlackBerry smartphone, according to early reviews before it hits North American shelves on Tuesday.

The pessimism of the reviews seemed to hit RIM shares, which were trading down 3.7 percent on Thursday afternoon, at $52.79 on the Nasdaq and C$50.81 on the Toronto Stock Exchange.

The 7-inch WiFi-only device is priced identically to Apple's 10-inch market leader and faces tough me-too competition from a slew of devices running Google's Android software.

It is a first step in a major product overhaul intended to reinvigorate RIM fortunes. But the lukewarm initial reception, coupled with an angry outburst from co-Chief Executive Mike Lazaridis, threaten to overshadow the coming-out party.

Most reviewers have been impressed by the PlayBook's much-vaunted capability to handle Flash websites and by its ability to show one high-definition image -- a movie, for instance -- on a connected TV, while doing something else on its own screen. Those are two things the iPad cannot do.

But more attention, however, was focused on what the PlayBook can't do.

The PlayBook will rely on a BlackBerry smartphone to access a cellular network or tap into RIM's popular BlackBerry Messenger chat platform. If you don't have a BlackBerry, it can grab a mobile connection via any smartphone or use the Web.

RIM has just shipped a BlackBerry product that cannot do email. It must be skating season in hell, wrote the New York Times' David Pogue.

On the other hand, the PlayBook's secure Bluetooth link does magnify a user's existing BlackBerry applications, negating corporate worries about leaking confidential information.

It was a question on the BlackBerry's vaunted security, and Indian government demands for access to the information that the BlackBerry protects, that co-founder and co-chief executive Lazaridis took umbrage with during a BBC interview this week.

That's not fair, this is a national security issue, he said before ordering the camera off.

RIM says the PlayBook and its brand-new QNX-based platform will be launched with around 3,000 apps, the third-party tools that have helped make Apple's iPhone and iPad so successful.

That number will grow in coming months as RIM adds support for Android apps and those available on its own smartphones, but is dwarfed today by the iPad's 65,000-strong library.

I got the strong impression RIM is scrambling to get the product to market, the Wall Street Journal's Walt Mossberg said.

RIM plans to add a video-chat app to make use of its front and back cameras soon after launch and key email and personal organizer features plus cellular connection later in the year.

But until then, I can't recommend the PlayBook over a fully standalone tablet, except possibly for folks whose BlackBerrys never leave their sides, Mossberg said.

Tech websites Boy Genius Report and Engadget took a much more granular look at the device, and zoomed in on what may at first glance appear trifling: a small and hard to operate power button.

It's impossible to find by feel and, once located, difficult to activate, Engadget said.

Reviewers also expressed concern that, days ahead of a launch that will define RIM's standing in the tablet market, the company was pushing out software updates to fix bugs.

The PlayBook of today is considerably better than the PlayBook of yesterday, which also was a big step forward from the one we were reviewing two days before that, Engadget said.

(Reporting by Alastair Sharp; editing by Peter Galloway)