Hey Tippy! Real-life personal trainer here, so take that for all of the experience -and biases- it's worth.
A good trainer is always worth it, especially considering your previous injuries. The biggest problem in our industry is the vast discrepancy between "good" and "bad" trainers; I have a 4 year degree, got the most rigorous certification in our field (the National Strength and Conditioning Association's Certified Strength and Conditioning Specialist, or NSCA-CSCS), and have 5 years of experience. The guy or girl who paid $500 to get a certification last weekend has the exact same job as me. Sounds like I got the short end of the stick, but it takes a college education's worth of human anatomy and kinesiology knowledge to get people stronger and injury-free... and then 10+ hrs a week from then on to stay up to date. We're closer to the medical field than a lot of people realize, and insurance companies are -hopefully- within a few years of providing coverage for "preventative health care" and "prehabilitation".
Your trainer sounds good, but ask if you can sit in on one of his sessions. Ask yourself these questions:
1. Is he hands-on with his client? He should be. Everyone has their own personal boundaries, but there's no quicker or more effective way to correct someone's form than by physically putting them into the correct positions.
2. Are his programs individualized? I once worked with a trainer who gave a bodybuilding arm routine to a 60-year-old woman fresh out of weight loss surgery. That would be a great example of ...well, the opposite of individualization. A shockingly high number of people, especially older people (and, unfortunately, "older" means 25+) have one or more injuries they need to work with or around, and require specialized soft tissue work, mobilizations, and strengthening exercises.
3. Is he more physical therapist than trainer? On the opposite of the "bro" side of training -the guy/gal who gives you their 6-pack abs routine no matter what your goals or contraindications- is the wannbe physical therapist. If 45 minutes of your hour's worth of traineing are spent with foam rollers, lacrosse balls, banded stretches, and mobility work, you're not getting what you paid for. We're not qualified to "treat" your pain; we just tell you how you can lose weight, get stronger, and gain flexibility without getting hurt. 10 minutes of soft tissue work and warm-up exercises, if well planned, should be enough, 20 if you're really jacked up, but remember why you're paying the guy and if you'd be better off getting physical therapy from the physical therapist he's shadowing.
4. Does he explain why he's doing this exercise/stretch/soft tissue mob? Granted, if you're observing a long-term client of his he might have already told them a hundred times, so you might have to ask. He should have a reason for everything he's putting into their program, from "we need to get her ankle more flexible to take stress off her knee" to "I want her back stronger, and she likes this back exercise". And does his explain it using plain English, or does he through as many big words as possible (she needs more ankle dorsiflexion to decrease valgus stress on her anterior cruciate ligament). The former means he wants you to learn, the latter means he wants to sound smart.
I know we're not cheap, but neither is knee surgery. If you learn a good warm-up routine and good lifting mechanics from this guy, it's a long-term investment rather than a cost. 5 or 6 training sessions, provided you let him know their purpose beforehand, should be sufficient.