Hey Miles, I've been using it for a while and I like it. It can reduce tape flutter a bit extending the highs for a couple of tapes worth by reducing friction. In practical terms though, I really don't notice that for more than a little while (as I get into the music those tiny differences just go by the wayside). The main reason that I continue to use it is that it makes keeping my heads and tape guides clean a breeze. I used to use Sound Guard on my records back in the day and it made the surface less prone to attract dust and the reduction in static made the dust that did settle on the vinyl super easy to remove. It's the same sort of thing with the Last head treatment. One little bottle goes a long way and it comes with some tiny foam applicators that just takes a couple of seconds to clean off any debris a snap. So it just makes keeping the heads spotless so painless that I simply do a quick wipe ( you can get most dust/debris off with a dry swipe with the applicators). I'll actually use the Last on them about every 5 tapes or so (maybe). I also follow cleaning with head cleaning solution with a treatment. I can see any debris on my erase head very clearly (since it's black) so when the dry/Last cleaning doesn't clean off everything, that's when I take out the big guns (varies a great deal with how old/nasty the tapes that I've been running across the heads are).
One hint;
The bottle comes with a plastic insert that prevents you from inserting the foam applicators directly into the bottle. Big PITA,.. you can pop that out, trim the bottom and reinsert into the bottle (it forms a seal with the cap). I guess it's the same bottle as the Last Stylus cleaner which you use with that little brush. Be sure to seal it up good (it drys instantly) and it'll last a long time.
Here's a thread with some tests I did with the Last products;
http://www.tapeproject.com/smf/index.php/topic,240.0.html