In terms of how Gorbachev got to the top of the party apparatus in the first place, the two reasons are he was seen as reasonably dynamic and effective, and that early in the piece he had some powerful patrons. In terms of the dynamic bit, Gorbachev was seen by other high level members as a hard worker who was good at solving problems. For example Yegor Ligachev relates in his memoirs an incident where the transport network ground to a halt due to adverse weather conditions, resulting in shortages all over the place. Gorbachev and others are portrayed as resolving the crisis by personally reorganising the transit schedules etc. as best they could. So to an aging Politburo who are generally aware that someone needs to get everyone working again and maybe shake things up, Gorbachev probably looked pretty attractive.

In terms of patronage, Gorbachev was 'discovered' by Yuri Andropov when Gorbachev was still working in the Stavropol Regional Committee. With patronage, CPSU members could rise through the ranks a lot more efficiently, particularly in the Brezhnev era when there was comparitively less movement in the ranks. Andropov is alleged to have tried to set Gorbachev as General Secretary before he died - the story goes that he had papers making his wishes known in a safe, but that on his death opponents (or at least allies of Chernenko) retrieved these papers and disposed of them. From memory this was first claimed by Arkady Volsky in his writings, but Volsky was Gorbachev's ally so it may not be an entirely true account. Other than Andropov Gorbachev is reported to have had an ally in Mikhail Suslov, and his later ascension to General Secretary was likely achieved with the support or at least lack of opposition from Andrei Gromyko.

I suppose Gorbachev's rural background may have also assisted in his rise to Secretary for Agriculture (replacing Fyodor Kulakov who was blamed for ongoing problems in Soviet agriculture) but it is more likely it was his support and perceived talents that enabled him to get the job, rather than just his background.

In terms of whether Gorbachev was an effective politician once he was in the top spot - mixed bag I guess. As illustrated above Gorbachev was able to form alliances in order to get to the top in the first place. For a while he managed to balance relatively liberal and conservative members of the leadership through promotions to smooth tensions and avoid the formation of an opposition block to his assorted reform attempts. Then again some of those conservatives he promoted ended up in the Gang of Eight who then attempted a coup, which doesn't speak highly of Gorbachev's choices or ability to keep his enemies in check. Even the 'liberal' figures turned on Gorbachev towards the end because of his perceived lack of commitment etc. Semi-related to his personnel reshuffles, early in the piece Gorbachev was able to engineer a bit of a soft purge by retiring aging high level party members, so I guess that was partly good politics but also just a reflection of the status of the party in 1985. In a similar vein Gorbachev probably used the Mathias Rust incident (Rust flew a light plane into the Soviet Union and landed on/near Red Square) as an opportunity to clear out some members of the military.

In terms of his policies clearly Gorbachev's record isn't too flash, however I don't think this was a reflection of Gorbachev being technically maladroit as a politician. He did manage the introduction of those policies fairly well. They instead fell down because they weren't necessarily good ideas in the USSR, or the consequences of them (ie. democratic reforms undermined party legitimacy, and hence Gorbachev's ability to manage events) just hadn't been thought through properly.

Finally - why was he allowed to stay on so long? Essentially the upper echelons of the CPSU were generally resistant to the idea of firing leaders. Khrushchev basically was the only one to get kicked out, rather than die in office, and that took years of Khrushchev essentially alienating everyone and capping it all off by threatening to completely restructure the CPSU. I think part of this reluctance to remove people from power from the Brezhnev era onwards was a mixture of rejecting the Stalinist era of purges, the replacement of 'respecting' party members/wanting stability and generally not wanting to set a precedent of people's jobs not being 'safe'. Essentially if you fire one leader, you might fire the next one and the next one and before you know it everything could be in chaos and who knows who could get fired next? Refering back to the liberal/conservative balance, another factor may have been an inability to achieve a consensus on who should replace Gorbachev. So essentially even though Gorbachev's policies were not hugely popular with the leadership, they lacked the willing/precedent or effective means to remove him.

Some references:

• Archie Brown: The Gorbachev Factor

• Yegor Ligachev: Inside Gorbachev's Kremlin

• Stephen Kotkin: Armageddon Averted

Gorbachev of course wrote and said a fair bit about his time in power after the fact, but I never got a good feel about the power politics from these so probably wouldn't be useful if you wanted to dig further into that area.