I have a question regarding the circle of 5ths and its application in lead guitar.

If I analyze a chord progression and determine that the key of the progression is G major, then I understand the 5th in the G major scale to by D. Correct?

In terms of applying that knowledge to a lead solo, what is the value? Does this mean that I can progress from the G major scale to the D major scale (albeit in a very slick manner!) over the G major progression that I cite above? Could I then continue to progress to the 5th of the D major scale (A) and begin to solo in that scale over the same progression?

Thanks for your help.
The circle of fifths is just a way of remembering the order in which the sharps and flats appear through the respective major scales.

In your example, G major has one sharp. F#. It's fifth is D, as you said. D major has two sharps. F# and C#. D majors fifth is A, as you said. A major has 3 sharps. F#, C# and G#. Find the fifth of A major, work out its major scale, and you'll find that it contains four sharps.

I'm guessing you probably already knew that. But if you didn't, there it is. To my knowledge, there isn't much of a use for it when it comes to soloing. Its just handy to have around when you're being pestered by key signatures.
Yeah i dont see any reason why not, it will add a bit more taste to your lead line, but as the circle is basically just a series of perfect cadences you will have to watch your accidentals in relation to the key signature.
Quote by hmmm_de_hum
the circle is basically just a series of perfect cadences

Going backwards (C - F - Bb - Eb ...) yeah. Going C - G - D - A - E is a series of imperfect cadences.

The cycle/circle is more a tool used in composition than improvisation. You follow the circle only because that is what you've decided that the chord progression is, not as a special technique in soloing. You could decide to use the G major scale and D major scale over any progression containing the G and D chords in any order.