It’s been about 5 months since I installed the Portech GSM gateway. It’s working well. I stand by the decision to buy a GSM-to-SIP gateway over one that presents an analog FXS interface. We’ve never had the kind of call quality issues that can result from small FXOs in Asterisk.
However, this morning I noticed that there are now some extremely low cost GSM gateways being offered on E-Bay. Some quad-band models as low as $99. This could be interesting if you just want to experiment with the technology.