Thule 591s and the updated 598 version are both absolutely excellent bike carriers. I've been using them to shift bikes around over many thousands of miles for years now and they've never let me down. A few thoughts from my experience: - Use them with proper slotted bars, ideally Thule. This makes them much easier to install and position accurately. Installing on square-section bars are, by comparison, a royal pain and, for the newer carriers, you'll need a separate fitting pack to even use them. - The 591 and, perhaps less so, 598 have security that is no better than window putty. My standing rule is that the bikes are *never* left unattended on them, but on the very rare occasions that they have, they've always been chained with a 15mm motorcycle chain and watched carefully. It's inconvenient, but I'm yet to lose a bike :-) - Keying them alike is a good idea, but here's a nice trick to save a few quid: if you have Thule bars and feet, they will have come keyed-alike with four identical lock barrels. If you leave the rack on the car most of the time (as I do), simply swap the lock barrels over, so the 4 identical barrels from the feet are on the bike carrier locks that are used most often, and the mismatched locks are on the feet. If you ever want to remove the whole rack it will mean needing 4 different keys (perhaps no bad thing!) but when putting bikes on and off, you'll only need one - If you have carbon bikes, it's well worth investing in some Thule Frame Protectors. They help distribute the point load from the clamps to avoid cracking lightweight frames. - Alternatively, look at the Thule 599 UpRide. It's significantly more expensive (£150), but does't touch the frame at all, only securing the wheels using a clever crossover clamp at the front, and a regular strap at the back. Highly recommended if you can justify it:
Their website says "coming soon" so potentially could be coming back in stock soon. Keep checking back (y)
I'm always hesitant to do things like this in case the thieving scumbags decide to just mess up my car in order to get the roof bars off. I'd rather just lose something than lose something and have to deal with fixing whatever they broke to get to it. You're definitely right about not leaving these things unattended, too many immoral opportunistic people out there.
You're going to be able to rip a bike out of any of them truth be told- they are only Aluminium & plastic. I ran a chain through all the bike frames and under/around the actual roof bar when I left it in the car park at the services for half an hour. If they have an allen key on them they can take the bars off, but it's more about making it difficult than making it impossible, as with most crime prevention. This is where not leaving them on the roof for ages unattended is also a wise strategy.
I have two 598s and two 591s on my car - had to get this combination to fit four bikes. The 591s are decent but the 598s are great - really easy to use and very secure. Just completed a 1500 mile road trip with 4 bikes using them - no issues! Highly recommended at that price. If you have multiple carriers and / or Thule roof bars, I’d recommend getting the same barrel for all locks so you only have one key for everything. Also, Halfords price matched from the web when I bought mine about a year or two ago and they fitted everything for me (for a small additional charge).