I've had my Slipstream for a couple of thousand miles, and it is a great bike. I, like you, test rode several recumbents, and this one stood out and just "felt right". I think Longbikes has this bike really dialed in.
In situations where (1) the bike is fully loaded for touring and/or (2) you are doing long miles on quiet country roads, the Slipstream is in its element.
Some other comments:
(a) There are certainly faster bikes out there. I think it is probably 1.0 to 1.5 mph slower than my bike-shop-quality recreational road bike. (I bought the Longbikes for the reasons above, however, and I'm not competing with anyone, so the speed difference isn't that important for me.)
(b) I've not found a hill I couldn't get up (although I'm not really looking very hard, either). That said, I think a grannier granny gear would be great to have. I think my low gear is a 30/30. A 32 or 34 on the back would allow me to spin a little more on the hills.
(c) This bike is a little squirrelly on gravel roads; I suspect because of the 20" front wheel and the weight distribution.
(d) I do 99% of my miles on quiet country roads. Being a LWB, I suspect if 99% of my miles were in the city that I would either get a SWB or, more likely, even a "wedgie". (Apologies to my 'bent brethren, but it's true.)
(e) I love the USS. This bike is dialed in and isn't twitchy at all, especially at speed. The USS also gives you the opportunity to answer the many questions about "How do you steer that thing?".
(f) I never had any muscle issues when I first started riding it, unless "hip flexor" is another term for "tail bone".
(g) I just returned from a 6-day, 470-mile trip on backroads with some hills and camping in state parks each night. The bike performed extremely well. My long day was 115 miles, after which I was tired, but not crippled.
(h) Regarding transporting, I have a minivan and it fits well inside.
I hope this helps. For loaded touring and long miles on quiet county roads, the Slipstream is wonderful. It's like sitting on my back porch, except the scenery keeps changing.