Mainline Protestants would say without hesitation that Catholics are Christians, because we define Christians as baptized people who profess faith in Jesus Christ, or more strictly as those who believe in the Trinity and the Incarnation. We consider your Communion to be one expression of orthodox Christianity.
Some very conservative confessional Protestants would say that the Roman Communion ceased to be Christian (in the same sense that we might all say that Mormons are not Christians) at some point in history, usually the Council of Trent, which conservative Lutherans and Calvinists and some other Protestants accuse of having “anathematized the Gospel.”
More common among evangelicals is a use of “Christian” to mean “someone who has living faith in Jesus Christ.” Many evangelicals hesitate to say that members of any institutional Church are all Christians, because from their perspective this is a statement about whether people are in a state of grace. Obviously Catholics do not think that all Catholics are in a state of grace! However, it gets trickier, because many, perhaps most evangelicals think that some Catholic teachings tend to obscure the Gospel and get in the way of people having a living relationship with Christ by faith. Generally evangelicals would admit that Protestant denominations do the same thing. They see the job of the Church as the proclamation of the Gospel without too many qualifications or complications. Naturally they think that they, evangelical Protestants, do this best

. And they may be heard to express a concern that many people in more traditional churches (particularly churches that have infant baptism, or a formal, liturgical style of worship, or a hierarchical organization, etc.) may be content with the rote practice of “religion” and thus lack the living faith in Christ that makes someone a “real Christian.” This is often what evangelicals mean when they say things like “some Catholics are Christians, and some aren’t.” They would say that the same is true of Protestant denominations, but they generally and understandably think that their own churches are likely to have a relatively higher proportion of “real Christians,” over against Catholics (except for Catholic churches that really emphasize personal faith, such as parishes influenced by the charismatic movement) or non-evangelical Protestants. This does not mean that these evangelicals consider Catholicism to be an invalid form of Christianity. It has to do with their rather idiosyncratic use of the word “Christian,” combined with their doctrinal disagreements with (and practical concerns about) Catholic doctrine and polity.
Edwin