I was user Jay on Locust (tried Jay3gsm but I wanted a shorter name, so I could use the extra few characters for my messages!!) I used to mod on the 3G, mobilechat and mobilenews channels, plus there were quite a few more mobile focused channels, mainly surrounding Orange as at least 95% of users were on the Orange network at the time.
Some months I couls send 700+ SMS to Locust and I know a few people whou would send even more! It was great when a GPRS gateway was set up so you could post messages that way but still receive via SMS. Saved me a small fortune. Back then the best GPRS phone was probably the 6310i, the 8310 came to market shortly after, and the 7650 was released shortly before Locust started to decline to Orange imagineering pulling its support.
I recall the service went to IM, where you could login via Yahoo messenger (MSN was always too unreliable) and read all messages that were sent whilst you were offline (I used agile messenger on my phone for this, worked very well) or otherwise I'd use the two way GPRS gateway, but over time as peoplel ost interest it became unreliable.
I think for most people back in those days the simplicity of SMS is what made Locust easy to use. Now, though, with people more tuned in to browsing, and with 3G access, etc, a Locust system could work again. I'm certainly interested.