Pros:
A pretty complete GT3 and GT4 experience.
Pretty good physics and adequate FFB. Update 1.8 added much but still somewhat inferior to Rfactor2.
Great reproduction of tracks. Standard of tracks and cars more uniform and often better than what's found in Rfactor2.
Pretty good telemetry support, works well with MOTEC. Some variables are missing by design, which detracts from the experience. Setup is VERY important for on-track performance. Setup interface is more user friendly than RFactor2. Monthly challenges are motivational and great practice for online races (that demand proper setup, fuel management, tyre management and an eye to changing weather)

Cons:
Safety Rating system not very well developed, so cannot recommend online racing on public servers. Both players involved in contact get penalized for any accident, regardless of whose fault it is. This makes it hard to accumulate safety rating to get access to servers that actually race clean, and once you accumulate it you are wary to get docked.I will admit that the official competition servers which requires high safety rating races somewhat cleaner that the rubbish public servers.

Car control and consistency rating are nice learning tool, but discourages practice with new cars, tracks and setups if you care to maintain you overall rating. It drops rather quickly unless you drive perfect from the getgo. Then you have to grind your best track in your main car to get it back up. Edit: Ratings can be suspended from options menu, which solves this problem. Edit2: Oh yes, turning of ratings disables recording of your fastest laptime. Why is that? How come active ratings are a prerequisite for documenting fastest lap? I mean, the hotlap functionality is completely unrealistic, the car resetting damage, tyre wear, temperatures, minimal fuel load every lap. Show some consistency here, kunos.

Verdict: Great ambiance. Complete GT-package with cars and tracks. Physics and FFB much improved since launch, but still somewhat inferior to Rfactor 2. Fair competition servers. Godawful safety rating system. Deplorable public servers.

Edit 2022: Changed recommendation from "NO" to "YES". Not perfect, but the most complete package for drivers interested in GT3.

Edit 2022 v2.0 - Safetyrating system still broken. Changed recommendatino to NO.

Edit 2022 v3.0 - Safetyrating stiill very much broken. Safety rating hovering in the mid-90, you get docked nearly 1 full point of safety rating per contact, regardless whose fault it is. Every single point may take several hours of "work" to recoup. Unacceptable, and very much a detriment to enjoying this otherwise good simulator. The REAL GT3 world challenge championship is awarding way less penalties for contact during real-world races.This along with superior physics makes Rfactor 2 a more attractive proposition for both sim-racer purists and gentleman racers. When will Kunos learn?

Edit 2022 V4.0. Yep. Safety rating still broken. Now high 80s after a high watermark at mid 90s. I got replays showing me on the racing line, driving in predictable manner and being tapped by AI trying to overtake. Single contact results in multiple OBWPs further docking my rating through no fault of my own. Disgusting.