I agree with the previous thoughts in that few, if anyone here, is qualified to offer insight about such thoughts. I certainly don't feel comfortable passing judgment on police officers and their techniques, as they have some of the most difficult of jobs. However, I do believe that, as with any important job, police officers should hold themselves and their colleagues to the highest of standards. Police officers should be aware of ensuring that they are bound by the Constitution's promises of procedural due process afforded to all and that all necessary precautions and steps are taken to ensure that the execution of the law has been done correctly and with the most transparent of attempts. Failure to do this can be criticized.
Go to ten different major cities, talk to ten different people in each and get 100 different responses. Every person's reactions to and feelings about their local law enforcement officers' effectiveness is going to be different, and rightfully so. There are effective police officers and ineffective ones, just as there are effective and ineffective doctors, teachers, politicians, etc.
But you asked for my opinion, so here it is. I do feel police in general do a good job in solving crimes. They are not perfect, nor is the practice of law enforcement, but I think they do a good job with limited resources and difficult working conditions for low pay.
Line ups can be reliable if they are done correctly. They can also be quite biased if they are not. Ties back to the first part of your question and whether or not the police are effective or not at doing their jobs.
I do not think that many of us have the necessary background to talk intelligently about whether the police do a good job of identifying criminals and solving crimes. We just don't know enough about the inner workings of the system. For example, it is rare to hear about someone who is convicted of a crime and then found to have been innocent. I mean, it happens, but the numbers are tiny compared to the number of people who are convicted.
So does that mean they do a good job? We can't know that. I would imagine that most of the people who are convicted would not really have the ability to challenge their convictions. It takes a lot of resources to do that, so there are surely cases we don't hear about where innocent people are convicted but where their innocence can never be proven.