I bought my first LP12 in 1982 and have owned one for most of the time since, though I have also owned other tables including several Rega P3's, the Roksan Xerxes, Roksan TMS, Michell Orbe SE, Avid Acutus, Spiral Groove SG2 and, currently, a Spiral Groove SG1.1.
I recently updated my 1989-era LP12 from the glued subchassis Valhalla to a Kore with a Lingo. I retained my Rega RB300 during the update but moved from a Shure V15 Vxmr to one of my two Koetsu Urushi Vermillions. Of course, it is difficult to know how much of the improvement I experienced owed to the change in cartridge versus the subchassis versus the motor and power supply. Suffice it to say, however, that while the Linn is not as good as my Spiral Groove SG1.1, Triplanar VII uii, and Koetsu Urushi Vermillion, the differences are more subtle than dramatic. This encouraged me to upgrade my LP12 further, though I have not yet accomplished that.
In my experience, there are a few observations I would make regarding the LP12.
With an LP12, everything matters, starting with the surface upon which you place the turntable, the degree to which screws have been tightened throughout the turntable, suspension set up and even whether the dust cover lid is up or down when playing. To be fair, other turntables may be just as subject to influence of various, similar factors. Good set up is good set up regardless of turntable. The LP12 may, however, be especially subject to the "tweaky" and "difficult to set up" reputation because the LP12 has a suspension that can be challenging to tune. What, exactly, does a "perfectly pistonic bounce" look like, exactly? (Hint: It depends upon how and where you tap the platter.)
While the suspension alone does not create all of the differences in LP12 performance based on set up, it is something that can puzzle and frustrate less experienced users. Getting the help of a good dealer is of great importance when setting up a Linn: Thomas O'Keefe in Michigan, Rick Duplisea in Colorado and Peter Swain in the UK come to mind as leading LP12 experts.
The LP12 is hardly a single model. A Majik LP12 selling for $4,200 or so does not perform at a state of the art level as may a Klimax LP12 that sells for more than $20,000. It is a remarkable product in that owners can, piece-by-piece, take a competent base-level turntable and transform it to a very refined and expensive turntable as funds allow without needing to buy and sell turntables as one chases improved performance.
Some liken the LP12 to a Porsche 911. The products both date back more than 40 years and both have been refined and refined and refined. Perhaps Linn would have better foregone the LP12's suspended subchassis and maybe Porsche should have mounted the engine in the middle of the car, not behind the rear axle. But maybe after having started in suboptimal places, each manufacturer has continuously improved the product to the point where each can compete with even more expensive and quite competent products.
I recently met Linn's representative responsible for dealerships in Europe and he assured me that anything, anything that one could imagine doing to improve an LP12, Linn has tried. It is a substantial engineering company with 150 to 200 employees. A dealer with whom I spoke recalled a factory tour he took years ago where he saw approximately 200 different belts that Linn was experimenting with for the LP12. Linn has taken very seriously (obsessively?) the opportunities to optimize all aspects of the turntable.
With approximately 150,000 LP12's having been sold, Linn has an incentive to keep experimenting and improving the LP12. I don't think that any other turntable manufacturer serves such a large installed base and thus has neither the prospective market, nor the resources, to continue to work on their tables as does Linn.
Given that the LP12 is more than 40 years old, there are a lot of LP12's out there in pretty dismal condition. Bearings have been run without oil (lots of them), LP12's have been transported without removing the platter, thus risking damage to both the bearing and suspension. LP12's have been set up in all sorts of suboptimal ways by well-meaning but inexperienced users, thus greatly diminishing the performance people have heard. In short, your mileage may vary.
Getting an LP12 carefully set up by a competent dealer or experienced user will likely provide a very good turntable and, at the highest level (Radikal, Keel, Ekos SE) will provide sound quality rivaling the best turntables. (Linn concedes nothing to any other manufacturer; they claim that the LP12 is the best turntable in the world regardless of price.) At the same time, a damaged table set up poorly that carries a so-so arm and indifferent cartridge can provide a not-so-great listening experience.
I am obviously an LP12 fan. If one intends to listen to a turntable for years into the future, it seems like betting on Linn and the LP12 is a decent idea. With 150,000 LP12's in circulation, there will likely be people out there to support it. Linn is a substantial company with a succession plan already unfolding with Ivor Tiefenbrun's son now running the company. Many smaller turntable manufacturers may not survive as long nor have the economic incentive to support their tables sold in much smaller numbers. Furthermore, an LP12 is likely to be improved upon going forward. Linn has improved the LP12 continuously and, if you believe Linn, will continue to do so.
While I am an LP12 fan, I sympathize with people who have had less than great experiences with LP12's. For reasons I have addressed, they aren't all great. Ivor Tiefenbrun, Linn and LP12 fans have an often well-deserved reputation for arrogance and an unwillingness to acknowledge that other turntables can produce good sound that rivals and may sometimes exceed the LP12. Idler wheel Garrards and Thorens can be terrific if tuned and maintained with a degree of care comparable to that which LP12's need. Direct drives from Technics, VPI and Brinkmann can also produce great results. The LP12 has no monopoly on great sound. But the list of companies who have claimed to have made turntables "better than a Linn" is quite long, far longer than the list of companies who have made the claim and remain in business today.