I wrote about this quite recently here. The below is a tweaked version of that comment.

One of the big shifts in Soviet historiography since the fall of the Soviet Union has been from a reliance on qualitative sources to a more quantitative approach. A problem during the Cold War was that historians just didn't have much data to work off. The estimates assembled drew on a wide variety of literary sources and were often informed by intelligent guesswork.

That's still a useful artform because Soviet data is never straightforward but the opening of the archives has given us a firmer evidence base. The figures culled from the various reports/accounts/documents may not be perfect or complete but we at least have an idea of what numbers the Soviets themselves were working off.

One of the first big questions to be argued over was the number of Soviet victims of repression. This had been running throughout the 1980s but the tension between pre- and post-archival estimates exploded into controversy in the 1990s. The key paper in English was Getty et al's 1993 Victims of the Soviet Penal System in the Pre-war Years. This pulled together the NKVD's figures for the Gulag population and substantially revised downwards several of the older estimates. Robert Conquest, poster boy for the older figures, did not take kindly to this: he argued for years, particularly with Stephen Wheatcroft, as to the validity/trustworthiness of the archive figures. Nonetheless, the latter are, with suitable revisions, generally accepted today. At least as a base.

The relevance of this to your question is that the archive figures strongly challenge many elements of the Gulag narrative from the more literary/memoir sources used by Conquest. They reveal that approximately 14m Soviet citizens passed through the camps with a peak population of about 2m (1953). They also show that 'political' prisoners (and here we have to be careful about Soviet categories) were never a majority of the population and that sentences were often relatively short at 3-5 years. Basically, they paint a picture of a much more fluid camp system than had been assumed: people moved in an out of the Gulag on a regular basis and for a variety of reasons.

This stands in contrast to Solzhenitsyn's picture of around 50m passing through the camps and a peak population of 12-15m. This was much more static picture of dissidents being sent to rot in Siberia for decades. This undoubtedly happened to some but Solzhenitsyn's intellectuals were not representative of the general population and their experience was not shared by all victims of the Gulag.

Hence the tendency today, which is not uniform, is to treat Solzhenitsyn's outputs as the literary and political works that they are. They're not a comprehensive survey of the Gulag system but remain valuable accounts of life within. I think it's very much worth reading them (particularly One Day in the Life) but as source material they need to be treated with caution. Particularly when it comes to generalising across the entire network of camps or talking numbers.

Sources:

Figures are from Wheatcroft's Scale and Nature of German and Soviet Repression and Mass Killings, where Solzhenitsyn's estimates are considered alongside others and the archival evidence.

[Edit: As a quick endnote, none of the above means that qualitative sources have been diminished in general. Access to the archives has produced personal diaries, police reports, etc that have greatly enhanced our understanding of Soviet society. But in those controversial cases where there are numbers to argue over - eg the economy, repression deaths, population fluctuations, etc - then the archive documentation has allowed a more data-driven approach. Of a tentative sort.]

Well, you can of course consider his overall experience reliable. But don't trust anything about numbers, he didn't have access to archives and documents, so they are based only on rumors and personal thoughts. And he clearly hated Stalin's USSR so his judgement was very clouded.

Here in Russia we read "Gulag Archipelago" in school literature classes as a major russian book. But it's considered as a depiction of that side of Stalin's era, not as some kind of historical source. It's not a memoir or diary it's still a fiction novel based on real experience.

By the way, many myths about USSR are based on Solzhenitsyn's fiction. I think, it's result of Cold War idiology battles.

I do know that Solzhenitsyn's history of Jewry in Russia, Two Hundred Years Together, is roundly condemned by historians for its glaring inaccuracies and for essentially being an anti-Semitic tract par excellence. So it's clear that historians take exception with much of Solzhenitsyn's work as a historian.

You have to understand that Solzhenitsyn was an intellectual heir of a long line of Slavophile and Pochvennichestvo arch-reactionary and profoundly anti-Western Russian philosophers.

I don't say this to condemn all of his work, certainly not the literary quality, or to discount every qualitative experience, but you bag to understand that all of it must be read in this light in order to understand where it's coming from.