The Anderson-Botham comparison is interesting, because I reckon Anderson will finish his career with more wickets at a better average than Botham. I think when analysing players there's an inherent bias in favour of players like Botham who start their careers brilliantly and then fade away compared with Anderson who start their careers badly and become very very good.
With Anderson's "first phase", I often think that part of the problem was that England were too impatient with him. He struggled in his second series against South Africa, but on the rare occasions where he actually had a place for an entire series (West Indies 2004, India 2007), Anderson did ok. Not outstandingly well, but decently enough. But the rest of his time in the England team was one off games here and there (Sri Lanka away in 2003 and 2007, South Africa away in 2004/05, India in 2006) where, with the exception of Mumbai, he did badly. In Australia he should never have been picked after his injury woes and he did about as badly as you'd expect, although he'd found a bit of rhythm by Sydney and bowled pretty well there from memory.
Phase 1 of Anderson's career - from debut to the end of the first Test in New Zealand in 2008, saw Anderson play in 20 out of the 64 Tests played by England in this period. His career figures in that period aren't terribly flattering - 62 wickets in 20 Tests at an average of 39.20, SR of 62.7 and an economy rate of 3.74.
After the first Test in New Zealand, Anderson came back into the side as Peter Moores decided the likes of Hoggard and Harmison were done and that blokes like Anderson and Broad represented the future. From that 2nd Test up until the most recent Test played by England, England have played 54 Tests. Anderson has featured in 50 of them - the only Tests he's missed after his recall in New Zealand have been the 51 all out disaster at Sabina Park (where the England management decided not playing Swann and Anderson was a good idea), the away series in Bangladesh (can't remember if Anderson was injured or rested), and the 2nd Test v Sri Lanka at Lord's when he had a side strain.
Since his recall in New Zealand, Anderson's career is 50 Tests, 205 wickets @ 27.28, econ 2.94, SR 55.6.
Yet even this phase can be further split - between his recall and the end of the South Africa away series, Anderson whilst a much improved bowler still had the tendency to put in anonymous displays (taking no wickets in the final 2 Tests of the Ashes in 2009 or the final Test in South Africa for example, despite bowling pretty well in the first 3 Tests of both series). Since the start of the 2010 summer (which is almost half of Anderson's "phase 2" career), his stats read thus:
24 Tests, 111 wickets @ 23.36, econ 2.65, SR 52.8
If Anderson maintains or improves his current series average then it'll be the 8th series in a row where he's averaged under 30 (in fact his worst series in this time frame has been in the UAE against Pakistan where he took 9 wickets at an average of 27.66.)
He's been performing brilliantly for the last 3 years, and IMO is the closest thing England have to a peak level McGrath.