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Following Marc's suggestion, I'll make my comments into a more extended answer.

The answer is yes. Every unitrary representation of $G$ can be "integrated" to get a representation of $C^{\ast}(G)$ and every representation of $C^{\ast}(G)$ can be "separated" to get a representation of $G$ (the terms 'integrating' and 'separating' come from the more general crossed product theory, and are the functors between the said categories). These constructions are mutual inverses and preserve a whole bunch of properties, one of which is intertwining operators, therefore the categories are isomorphic. You could take a look at Proposition 2.40 in "Crossed prducts of $C^*$-algebras" by Dana P. Williams (who does it for only equivalences) or Proposition 5.5 in this paper (http://arxiv.org/abs/1104.5151) by some colleagues of mine (who do it a lot more generally, but they do specifically include the case for intertwiners which is what Sergio was asking about in the comments).

One small difference is that in both these references strong instead of weak continuity of the group representation is assumed. I don't think this is too much of a problem, as long as we can pull bounded operators through integrals (however we might choose define those). But do correct me if I'm wrong!

PS: As Andreas mentions, this isomorphism might not be particularly useful. For abelian and compact groups this is the case, but it does become useful in the general case!

We often want to answer the following question:

Can we write every unitary group representation as 'built up' from irreducible representations? (representations with no non-trivial invariant closed subspaces)

For the representations of abelian and compact groups we can go through the motions and show that this is indeed the case without ever needing to resort to this isomorphism of categories (take a look at the Peter-Weyl theorem for compact groups on wikipedia). However, for general a unitary representation on a seperable Hilbert space of a separable non-abelian, non-compact, locally compact groups things become more difficult but still works. To get a result, the rough idea is as follows: We transfer everything to the category of representations of $C^\ast (G)$, and invoke von Neumann algebra theory to show that every such representation can be written as a "direct integral" of irreducibles and then transfer back to the category of group representations to conclude that every such unitary group representation can be reduced to a direct integral of irreducables. Details are in Chapter 14 of "Real reductive groups. II" by Nolan R. Wallach

UPDATE: As Yemon pointed out things are not as simple as first thought, and what I remembered reading in Wallach is not what I actually read.

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Following Marc's suggestion, I'll make my comments into a more extended answer.

The answer is yes. Every unitrary representation of $G$ can be "integrated" to get a representation of $C^{\ast}(G)$ and every representation of $C^{\ast}(G)$ can be "separated" to get a representation of $G$ (the terms 'integrating' and 'separating' come from the more general crossed product theory, and are the functors between the said categories). These constructions are mutual inverses and preserve a whole bunch of properties, one of which is intertwining operators, therefore the categories are isomorphic. You could take a look at Proposition 2.40 in "Crossed prducts of $C^*$-algebras" by Dana P. Williams (who does it for only equivalences) or Proposition 5.5 in this paper (http://arxiv.org/abs/1104.5151) by some colleagues of mine (who do it a lot more generally, but they do specifically include the case for intertwiners which is what Sergio was asking about in the comments).

One small difference is that in both these references strong instead of weak continuity of the group representation is assumed. I don't think this is too much of a problem, as long as we can pull bounded operators through integrals (however we might choose define those). But do correct me if I'm wrong!

PS: As Andreas mentions, this isomorphism might not be particularly useful. For abelian and compact groups this is the case, but it does become useful in the general case!

We often want to answer the following question:

Can we write every unitary group representation as 'built up' from irreducible representations? (representations with no non-trivial invariant closed subspaces)

For the representations of abelian and compact groups we can go through the motions and show that this is indeed the case without ever needing to resort to this isomorphism of categories (take a look at the Peter-Weyl theorem for compact groups on wikipedia). However, for general non-abelian, non-compact, locally compact groups things become more difficult. To get a result, the rough idea is as follows: We transfer everything to the category of representations of $C^\ast (G)$, and invoke von Neumann algebra theory to show that every representation can be written as a "direct integral" of irreducibles and then transfer back to the category of group representations to conclude that every unitary group representation can be reduced to a direct integral of irreducables. Details are in Chapter 14 of "Real reductive groups. II" by Nolan R. Wallach