After months of talking the talk,

Verizon

(VZ) - Get Report

is finally walking the walk. Now, once more, the question is whether wireless users will stroll away from

Nextel

(NXTL)

.

After much talk and not a little waiting, Verizon is rolling out its new walkie-talkie service, featuring a so-called push-to-talk two-way radio feature that's aimed squarely at rival Nextel.

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Nextel's greatest advantage has been a service called Direct Connect that enables work groups and field service teams, not to mention teens and taxi drivers, to make conference call-type connections. That hugely popular feature has helped Nextel dominate its niche with business customers and deliver the highest per-subscriber revenue in the industry. Investors have been happy with the company of late as well, propelling its stock to a more than 200% gain over a year.

Now, with Verizon's offering, and

Sprint PCS'

(PCS)

own version due out any day, Nextel will face its first head-to-head competition for its higher-spending customers.

And while Nextel has long contended that its technology is superior to its rivals' best efforts, investors have been less confident.

Shares of the No. 5 cell-phone service have slipped from their recent highs despite Nextel's better-than-expected sales and subscriber growth, as well as solid profits. In midday trading the stock was up 36 cents, or 2%, to $17.63.

For Wall Street, the overhanging issue in wireless -- now that subscriber growth rates have peaked -- is how well companies can hang on to their customers, and keep them from shrinking their phone bills. The worry is that in a six-player market, differentiation is critical. Having a distinctive technological edge always helps, especially as the industry braces for a November rule change that will allow customers to keep their cell numbers as they jump to other carriers. Now Nextel's advantage is at the very least under pressure.

To be sure, Nextel has a very loyal following among business users and one of the lowest churn rates, tracking customer defections, in the business. But with rivals imitating Nextel's service, investors can't help but wonder how long it will take before Verizon and Sprint also imitate its success.