I have rented there twice, the first time a month or so after they opened, the other time was last month. Yes, they are off site, but if the timing is right it is not that bad. I will say that from the first time I rented to the last, they have upgraded the size of their shuttle, to one of the NIssan NVs and they now hold about ten people. That is usually ok, except my last trip, there was a group of about 15 from Europe and they took up the whole van. I got on because I was by myself and could fit in the passenger seat.
Their cars appear nicer and better equipped than other "off-site" companies. In Dec, I opted for E-Z instead of SIXT and regret doing it. I believe, it was the only mid-size rental I have ever had with manual windows!! Last month, at SIXt, I had one of the Scar class reserved at $13/day. I upgraded to an E-Class convertible for $20/day!! I am not sure what EZ uses for a luxury, but at SIXt you WILL have a BMW or Mercedes. If the price is anything close, I would take SIXt in a heartbeat.
The line at the station was not that bad. I had to wait, because I was by myself and had to wait for my luggage, while the group "leaders" of the other 10 on the bus had family members retrieve the luggage, while they took the first spot in-line. There were 3 lines open for check-in. I only have Android, but they have an I-phone app that you can use to check-in and then you would not have to wait in line.
The hold was, I believe, $1K or $2K, don't remember, it was higher because I had an E-class. They released it as soon as I turned in the car. I would also recommend not arriving at the same time as the flights from Europe do. It appears that since they are a German company, a lot of Europeans use them. They also don't pressure you into LDW, etc. EZ was BAD for that!!
I am renting from SIXt again at MCO in April, so I am happy with them. Hope this helps.