Honestly, I would go see one personally before you make the commitment. Get a quote for a Sterling product, then get a quote for a retro fit system. Personally, for the ones I had seen - I couldn't tell immediately they were retrofitted systems, but then after further inspection it was obvious. The price is what turned me off. For the same price, you can have a new tub put in, have the plumbing done correctly - but it will take a bit longer to have it done. I think those retrofit places do it in ONE day. I personally took a week to do my bath, put the drywall up and finish it, if you break down the timeline with just those in mind for the project.
BUT be careful, once you replace a tub - they often aren't the same size and you might have to redo your floor!
Personally, if it's an old home I would rather they rip the old sucker out, put down a cheap floor and replace the old gunky plumbing around the tub with a new faucet valve and faucet etc - than to have a cover up slapped over what could be a ticking time bomb.
For the 3 bathrooms I've done in the past 6 months, 3 of them were leaking at the valve or the drain. Floor damage was present as was black mold and rotting supports. I replaced all that, put in access panels for 6month - 1 year inspections of the work, and am very happy I chose to do it 'the hard way' rather than slap something over an old problem.
I think retrofit baths are a GREAT way to flip a house...put down some cheap linoleum floor, new $150 toilet (cadet 3 of course) and a off the shelf vanity from lowes or home depot that looks designer , coupled with fresh paint, new fan/lights - and you have a $9k looking job for about half that price.