Still at a minimum it is a good idea to have payment information set up on your account so that you can buy Lindens (Second Life money). With Lindens you can get a place to live, buy clothes, etc.. also, some venues do not allow avatars without payment information on file to enter.
Second Life offers incentives for visitors to become 'Premium' members. They currently are:
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A weekly L$300 stipend or allowance. This usually comes on Tuesdays, and you can 'spend' it for a lot of things. Some frugal avatars save it up and cash it out to help pay for their next renewal.
A one-time L$1000 sign-up bonus.
Priority entry when a region is full - Think of it as a VIP entrance. Use of Premium areas and experiences (mainly the Premium sandboxes).
Live-chat customer support (for what it is worth).
Premium virtual gifts - for example, last year they gave out holiday lights and decorations.
More membership slots and increased cap on instant messages.
Voice Morphing for those who use live chat.
The major benefit is the 1024 'free' tier allotment. This allows you to choose a Linden Home (subject to availability), or, if you prefer Mainland, to have 1024 deducted from your tier after you have acquired land from Second Life. (See "How Not To Buy Land" for more information).
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I have been in Second Life for years, and have almost always had at least one Premium alt because I wanted to contribute to the community, and I liked to 'own' land on the Mainland rather than rent. Recent events have left me 'virtually' homeless though and I'm not sure I'm going back.
Still, I don't want my sour grapes to dissuade you. If you can afford it I would recommend getting a Premium membership. Just don't confuse 'virtual' with 'reality'.
