Some months ago, I was creating a Windows-based controller to manage a modem bank using Java. After initially trying serial drivers from RxTxComm, IBM and Sun each with various problems in stability and operational behavior, I purchased serialio.com's SerialPort package.
I was immensely pleased when the package went in seamlessly with no coding changes (using the javax.comm.SerialPort layer), and more so that it has been working reliably ever since.
The original goal was to control up to 16 modems on a mid-level (circa 2003) system, thanks in part to SerialPort's implementation, the final product is confidently estimated to support at least 128 and as many as 256 modems, concurrently streaming data at up to 56Kbps.
I should have bought SerialPort at the outset, and saved myself considerable time and frustration. My future Java projects that require serial I/O will definitely use SerialPort.