Hi there
the transporter cabs are basically hartke's cheaper alternative to their 'normal' gear. I have this exact cab.
I use it for practice and small concert band gigs (driven by an swr bass350, with my MMSR5 and a sansamp/few other pedals), and for what it is (and how much it costs), i think it is a great cab. I tried a couple of other 210's around that price range but ended up with the kartke (GK backline was one that i previously owned and remember not liking as much). I'm not aware of any HUGE differences between it and the 210xl, however i do know that the 2.5xl (the one with the extra little tiny speaker) is supposedly much better (i've never actually tried one, im just going off reviews i've read etc). As far as low end response goes, if you play on really cranking your low B then you can forget about it. My advantage in using it with a concert band is that i'm not competing with two guitarists with marshalls and a full drumkit, the worst i get is an overly enthusiatic percussionist or a few tubas playing loudly, so even with a slightly limited low B response i can still compete with the other instruments.

As har as the high end goes, it does have the 'classic hartke' aluminium speaker type sound (hard to describe but if you've played one u know what i mean).
At the time I decided to by it because it is small, light, has good handles, and was good value for money.
Some would say its 'disadvantages' are that it has no tweeter, it has limited low freq. response (especially for 5 strings) and limited power handling. But, for the money, atleast some of these are to be expected?
If I were to buy two cabs, I would get the 115TP to go with the 210TP, just to give you a bit more low end response.
I am starting to get a few more regular 'smaller' gigs now (i.e. i am using my 'mini rig' slightly more often now), and if this continues I may upgrade to a higher quality small cab (markbass 210 traveller, eden 112 and aggie 112 are what i'm tossing up atm), but i think I would still keep this cab as a backup/home practice cab.
hope that helps
-Sean