What is your opinion of Solipsism?

Two parts: "I know the Noumenon of I" (meditation of Schopenhauer);

and, "I am the only one" (this is illogical; rather, one's Schopenhauerian knowing has no absolute certitude re otherness).

Descartes' solipsism was simply radical epistemological questioning; Descartes intuited or discerned Mind, God, the One, as informing and guiding Descartes' knowing of self. Hence, Cartesian "I think, therefore I am" is not true solipsism, as Descartes' "I think" reflects Mind, his God..

Schopenhauer's intuition of "will" was that of inchoateness, rather than Descartes' "The Father and I are of one Mindness." This "noumenal quality" is the extent of Schopenhauer's awareness of Noumenon, hence frames his "solipsism."

Therefore, it is evident that the awareness of solipsism is dependent upon the thinker's presence. If anomic, as with Schopenhauer (not to mention the Sartres), then solipsism is more pronounced. If a Descartes, Plato, Plotinus, or a Whitehead takes a radical epistemological turn, their ontological certitude moves beyond solipsism as a personal enclosure, for these minds understood God, Oneness, hence there is certitude per self (a typical non-radically-questioning philosophical position), with options towards scientific intermonadic community and/or I Am oneness of theology.

Kant's position was transcendentally philosophic: Man philosophically interpreting God (theology, metaphysics thereof) and Nature (science). Kant's distinction between phenomenon and Noumenon ---> science/non-solipsism per sharing protocols re 5-sense data and ---> God as ineffable (hence, a stage beyond Schopenhauer's solipsistic Noumenal meditation--that of God, Mind, which theologians and some philosophers encounter).

Schopenhauer's "noumenal solipsism" reflected his lack of awareness of (or conviction re) God as integral basis of Noumenon. Therefore, there is no real validity in taking up a solipsistic position; as "Amy" said, there is validity in science (and reasonableness in "I-Thou" theology/philosophy), and Kant's dualism disallows solipsism per his "awakening" re Humean solipsism (Hume ~ "I can't be sure of the causality of anything physical, and I deny God Is"), hence the need as perceived by Kant to bifurcate God, Noumenon, for the theologian/metaphysician, and Nature, science, for the intersubjective community of such workers. As Godel and others have shown, even the solipsistic position, at a reasonable level of axiomization, is necessarily incomplete. Thus we are left with Kant's science, Whitehead's process philosophy and theology, and direct insight of e.g. Plotinus ("One Mind Soul"); solipsism is seen to be both necessarily incomplete and practically irrelevant. Wittgenstein addresses this human impossibility of solipsism by his observation that there is no "private language." Ibn Tufayl's "Hayy Ibn Yaqzan" addresses the movement from solitude to wisdom, and this shows at least part of the spectrum of philosophical thought as one's solitary psychological awareness moves towards the Light of the One. The use of "solipsism" as an epistemological focus is seen in Descartes--questioning various paradigms or axiomizations.

Related:

"There's Something about Godel;"

"Return to the One: Plotinus's Guide to God-Realization;"

"A Philosophy of Universality;"

"On Certainty," Wittgenstein;

"Hayy Ibn Yaqzan: A Philosophical Tale," Ibn Tufayl.

Why are you talking to me? There is no me; there's only you.

It not a very healthy state to be in

There may be no truths beyond "I think therefore I am" but that will hardly tell you how to act and if you don't act you'll die. You can still hit your head on a beam you didn't see and be in pain. Solipsism has a very convoluted answer for how that happened (and doesn't provide you with a mechanism to avoid future beams).

If solipsism is all in the mind, how can you lose your mind

Solipsism is the only epistemologically valid position. That said it an extremely useless philosophy because if you actually believe in it you'll perform terribly.

There may be no truths beyond "I think therefore I am" but that will hardly tell you how to act and if you don't act you'll die. You can still hit your head on a beam you didn't see and be in pain. Solipsism has a very convoluted answer for how that happened (and doesn't provide you with a mechanism to avoid future beams).