Preaching to choir here but for my money this remains one of the most outstanding plugins within the Sonar arsenal.  Despite its age and outdated GUI, it is just magic!  I had a mix the other day that lacked the warmth and bottom end of my reference file.  I stuck this plugin into the bin and bang...my mix was saved.  Amazing...wanted to mention.

How does it compare to, say, Massey's Tapehead (another freebie)? And what is the difference between tape saturation and things like the Softtube saturation knob? 

Never tried Massey.  This has always been my go to.  Don't know the answer to your other question either.  Sorry.  Just know this pluin is VERY good!

I use Voxengo TapeBuss a LOT. Not really for making things sound like tape, but it has this Hi-Freq control that adds some sweet sweet sizzle to the material without being gross about it.

I use it on bass guitar buss on almost every single project. It adds grind and loveliness without harshness (if used with some sort of moderation). It helps the notes come alive in a way that I love.

I used to use the Cakewalk Tapesim a lot, but haven't since I got the Voxengo plug.

+1 for CW Tape sim - underrated IMO

I was surprised about how much the console mix and bus warmed up my mix... I think I tried the S type with about a 1.5 drive and it was really too much on some mixes.  I'm going to try it again using the A type and then I'll try the tape sim next.

There are two threads down in the Gear/software forum where the CW tape sim was compared this past summer to some other freebie and some more expensive tape sims.  You have to wade through many of the posts, but doing so give you a perspective on how good the CW tape sim actually is.  Here they are:
http://forum.cakewalk.com/tm.aspx?m=2618967
and
http://forum.cakewalk.com/tm.aspx?m=2616606

I was just trying this the other day myself.

They should update the plug-in to 64-bit and revamp the GUI - but otherwise keep it the same.

It cuts mid, adds highs and lows in a very sweet way.

They should update the plug-in to 64-bit and revamp the GUI

 
+1 one for *all* the FX series plugins. 

I find it interesting that it is still 32 bit without any stability issues.  32 bit plugs from other manufacturers in a 64 bit environment have been a major source of headache.