The days of free Internet access are back… sort of.

United Online, Inc. announced Monday that it will offer free wireless Internet access through its NetZero service, provided as a “loss leader” that depends on users upgrading to paid access to cover the service’s costs.

NetZero became familiar to most Americans in the 1990s when the company handed limited dial-up Internet access, paid for through online advertising that subscribers endured in return for getting the service for free.  But broadband costs considerably more, so as the transition away from dial-up turned into a stampede, NetZero faded into memories about as much as AOL signup floppy disks and CD’s.

But now the company is back pitching free access to “4G wireless Internet” with no strings attached, contract commitments, or overage fees.  But that does not tell the full story.

While there is no contract commitment, NetZero requires an upfront investment in wireless hardware — $50 for a USB antenna stick suitable for a laptop or $100 for a “mobile hotspot” that can deliver a Wi-Fi connection to other nearby devices.  The devices are for sale on NetZero’s website.

The “free wireless” offer is probably better described as dim sum — it comes with a 200MB monthly usage limit, which makes it suitable for basic web browsing and e-mail only.  Once your limit is reached, the service is cut off for the remainder of the month, unless you agree to one of several paid usage plans that range from $9.95 for 500MB to $49.95 for 4GB, billed monthly.

After 12 months, NetZero’s free ride is over unless you agree to continue with a paid usage plan.  It ends even sooner if you choose to upgrade to a paid plan anytime during the first year.  Once you do, you lose the option of switching back to the free plan.

Whether paid or not, NetZero users ride on Clear’s troubled 4G WiMAX network, which Sprint — Clear’s largest customer — is planning to eventually abandon for more advanced LTE.  The long term future of Clear, also known as Clearwire, is also up in the air.  The company has ceased investing in its WiMAX network and is making preparations of its own to switch to LTE 4G technology — incompatible with the NetZero hardware you will spend $50-100 to acquire.

Clear’s network has also received considerable criticism for its speed and performance.  Because it operates on much higher frequencies, Clear’s wireless signal has problems penetrating indoors, and has even more trouble where energy efficient window coatings are used, especially in the south.

While NetZero does, in fact, deliver the service for free, the upfront investment and potential service headaches limit its usefulness.  Light users may find free Wi-Fi, increasingly common in a number of businesses, more convenient, affordable, and faster than the NetZero alternative.