Sonar is the direct descendant (and may be able to open your old files?). Most recent version is 5 - its available in Studio and Producer editions - check it out at cakewalk.com
I have opened PA9 files in Sonar 1 and 3. I'm pretty sure Sonar 4 and 5 will too. However, once they are saved as a Sonar project, PA9 will not open them.
Cakewalk Pro Audio 9 is a perfectly good recording program if you are new to PC recording and have a Pentium 3 or older PC. If all of these apply, I say go for it. I found it on Ebay for $10 plus $5 for shipping. Sounds like a good deal to me.
If you can get Cakewalk AudioFX1 to go with it you'll be in good shape.
PA9 is awesome especially for midi sequencing. I would seriously consider though looking for a version of SONAR. I know people are selling older versions on the net very cheaply, and even SONAR 1 is a huge step up from PA9.
I use PA9 to do any MIDI and to lay my tracks down and do any necessary bouncing, simply because I find the interface is easier to use to do that kind of stuff. But come serious mixdown time, I pull it into S4PE.