I have a good amount of experience with Lantus. It's my preferred insulin. The only other insulin I like is humulin. Levemir is okay if you can't get lantus but it's not as powerful of a muscle builder. Lantus has some cross interaction with IGF-1 receptors on paper, which seems to apply in reality since I gain more LBM with lantus than with levemir.
Insulin use is NOT healthy, IMO. Both very high peaking and sustained elevations in insulin are supported by lots of research to directly correlate with poor health and expedited aging.
Then again, AAS use is considered by many to be unhealthy also. It's all really beside the point because insulin and GH are very powerful body changers when combined, and many like the idea of using them together. I still think that periodic insulin use with more moderate AAS use is better than no insulin use and abuse of AAS.
Lantus doesn't peak much if at all. It's a basal insulin. Your basal insulin levels will be elevated as a result of using it, however you will still produce a bolus at a meal in response to elevated blood glucose/amino acids. Over long periods of time, having elevated basal insulin will decrease insulin sensitivity some and also potentially cause other damage. This will come on a lot slower than if using a higher peaking insulin like Humulin/Humulog several times per day, consistently. Fat gain should also be less as there is likely to be less total insulin in the blood. Regarding the first issue (insulin sensitivity), this may be mitigated some if the Lantus is activating IGF-1 receptors as a different path of glucose disposal, however I wouldn't count on it completely.
I used lantus for 4-8 weeks at a time, in "blast" fashion with AAS and GH (10iu/ED when on Lantus) and I was able to grow lean, big and did so quite quickly. When I had been interested in competing, I was able to use the above protocol with moderate doses of AAS to get to about 250lbs at 5'10" at 22yrs old. I was also about 8% body-fat, so quite lean. I had room to keep growing, but chose not to as other priorities came about in my life.