I've been attuned to Reiki and received first degree (level 1) training. I've also used Reiki to help heal people before, and have had Reiki done on myself as well, and have also been able to detect where physical/emotional problems are on someone based on feeling and sensing their energy field. Long distance Reiki works under the same principles; that the Reiki practitioner facilitates the flow of Reiki and that Reiki does the healing. My view on it is that Reiki is sacred healing energy that comes from the divine, and I believe it works and I've witnessed it working.
For me personally, it took experience and time before I was really receptive to energy, and then when I started to learn Reiki I was already able to physically feel some of the energy sensations. But for others it takes more time to feel/sense things, others it takes less, but I believe its just a matter of being open and receptive to learning it and recognizing that Reiki is real and that healing does take place. I also have posts about my Reiki experiences in this
thread. I truly believe in spiritual healing, so if you want to go get training in it to learn Reiki then go for it. I think its awesome and I'd recommend this type of healing to anyone.
Edit:
Depending on how receptive you are to energy or spiritual things, you may feel the attunement more or less intensely. But the attunement increases a person's ability to facilitate the flow of Reiki, as well as energy in general. So you may find that you can sense energy better afterward. The Reiki teacher uses the Reiki symbols during the attunement as well.
But the training part is just as important as the attunement, if not more important in my opinion because thats when you learn energy sweeping, dips or blocks in someone's energy field, the hand positions, ect. The attunement simply opens you up more to using Reiki and sensing energy, and is not required to use Reiki. Someone without an attunement would still be able to do Reiki, but usually (there are always exceptions) it's not to the extent of someone who has had the training and attunement.