I'll throw in my two-cents worth regarding my experience with Convenia. I have a 16 year old, hyperthyroid cat named Rupert with a chronic sinus infection that's been ongoing for the past 18 months. A sinus culture suggested nasal lymphosarcoma but all these months later it has shown no sign of progressing so my vet suspects it's quite likely a deep seated bacterial infection lodged well inside the sinus cavity and perhaps not cancer after all. It responds well to antibiotics but, till recently, symptoms would re-occur once the antibiotics wore off.
Before the cancer diagnosis, I had struggled to give Rupert oral antibiotics for several rounds (we know how hard it is to get that horrible liquid pink stuff into a squirming cat). Once we had the cancer diagnosis from the lab (it was vague; it said the culture "indicated a condition consistent with nasal lymphosarcoma"), the vet suggested Convenia to keep Rupert comfortable, thinking it would be only a matter of time till the cancer progressed. So for more than a year, Rupert received an injection of Convenia every 6 weeks. It seemed to be a wonderful treatment for keeping his symptoms under control -- after each injection he would have 4 or 5 symptom-free weeks and then the sneezing and discharge would start again, and soon after that he'd get lethargic and miserable, so we'd go back for another shot. However, as of this moment, he hasn't had an injection for 8 weeks and there are no signs of symptoms flaring up, so I'm crossing my fingers!
I know there are all kinds of reasons why long-term treatment with antibiotics isn't ideal. Initially, because we thought he had cancer, it seemed like a good way to help keep him comfortable. As time wore on, my fear was that he would develop a resistance to the antibiotics. I guess only time will tell as to whether the problem is permanently gone, but Convenia offered us a way to keep Rupert's quality of life high without the trauma of oral antibiotics.