How are Rosencrantz and Guildenstern characterized? How are we supposed to feel about them?

In Act 2, Sceene 2, when Hamlet sees them coming, he refers to Rosencrantz and Guildenstern as, "These tedious fools." They have been sent for by the King to spy on Hamlet and report back to him, and Hamlet knows it.

They are, just as Hamlet characterizes them: tedious fools, meaning that they are dull, boring, monotonous nonentities, and they're none too smart.

They are such nonentities, so interchangeable as individuals that this is exchange is usually interpreted so that the King is corrected by the Queen as to who is who:

KING:

Thanks, Rosencrantz and gentle Guildenstern.

QUEEN:

Thanks, Guildenstern and gentle Rosencrantz.

Even by their own admission, they don't amount to much:

HAMLET:

My excellent good friends! How dost thou,

Guildenstern? Ah, Rosencrantz! Good lads, how do ye both?

ROSENCRANTZ:

As the indifferent children of the earth.

GUILDENSTERN:

Happy, in that we are not over-happy.

On Fortune's cap we are not the very button.

HAMLET:

Nor the soles of her shoe?

ROSENCRANTZ:

Neither, my lord.

They are nothing but tools to be used as other people wish them to be used. First they are used by the King, and then they are used by Hamlet. And as all tools, when they are no longer useful, they are easily discarded.